Black Silicone Seam White Separating?

[BloopFish]

I see this in only the front left corner seam. Disappears when the tank is only 1/8th full, but starts slowly appearing when it is 1/4 full and the picture is at around 1/2 full. Brand new tank, though I think it may have been in the storage for a year or two. What is notable is that I did not notice this until I filled the tank. However, now that I look closely with a flashlight against the silicone - I see the areas where the silicone appears to have "stretched" apart. I built the stand for this tank and it is level and its on plywood on top of a stand made out of 2x4s and 2x6 on a 36" length tank with a trim and bracing. Could it be due to the stand being warped? I tried putting a 5mm yoga mat underneath the tank and it didn't change anything. Honestly lost as to what to do since this tank was on clearance and there was no other one left to replace.

 

[UncommonSense]

I see this in only the front left corner seam. Disappears when the tank is only 1/8th full, but starts slowly appearing when it is 1/4 full and the picture is at around 1/2 full. Brand new tank, though I think it may have been in the storage for a year or two. What is notable is that I did not notice this until I filled the tank. However, now that I look closely with a flashlight against the silicone - I see the areas where the silicone appears to have "stretched" apart. I built the stand for this tank and it is level and its on plywood on top of a stand made out of 2x4s and 2x6 on a 36" length tank with a trim and bracing. Could it be due to the stand being warped? I tried putting a 5mm yoga mat underneath the tank and it didn't change anything. Honestly lost as to what to do since this tank was on clearance and there was no other one left to replace.

Interesting! This looks like either an insufficient quantity of silicone was used to fully fill that joint during assembly, or the silicone is indeed stretching an alarming amount… (I’m leaning towards the former though; can you see the slightly wavy line in the silicone seam? — like a bead of silicone being laid by hand then squished between glass panes…)

Is this air gap visible with the tank empty? Or does it disappear like the silicone has shrunk back into place?

Also, thanks for the excellent photos! — these things can be tough to accurately diagnose off grainy images!

 

[BloopFish]

Interesting! This looks like either an insufficient quantity of silicone was used to fully fill that joint during assembly, or the silicone is indeed stretching an alarming amount… (I’m leaning towards the former though; can you see the slightly wavy line in the silicone seam? — like a bead of silicone being laid by hand then squished between glass panes…)

Is this air gap visible with the tank empty? Or does it disappear like the silicone has shrunk back into place?

Also, thanks for the excellent photos! — these things can be tough to accurately diagnose off grainy images!

You can see what appears to be sort of like stretch marks on the silicone if you shine a light at the precisely correct angle and look very close. It isn't an air gap when the tank is empty either. I took another picture when the whitening disappears, with a highlighted version as well.

 

[UncommonSense]

You can see what appears to be sort of like stretch marks on the silicone if you shine a light at the precisely correct angle and look very close.

I do see those! Like it’s almost torn there, which really should never happen… any silicone worth using can elastically deform +-25% at dead minimum…

It isn't an air gap when the tank is empty either. I took another picture when the whitening disappears, with a highlighted version as well.

I don’t quite understand this part… are you saying the air (or maybe just light through a thin film of black silicone) gap is actually closing when you empty the tank? This would point towards a remarkably defective silicone!

 

[BloopFish]

I do see those! Like it’s almost torn there, which really should never happen… any silicone worth using can elastically deform +-25% at dead minimum…



I don’t quite understand this part… are you saying the air (or maybe just light through a thin film of black silicone) gap is actually closing when you empty the tank? This would point towards a remarkably defective silicone!

From what I can tell, this white line doesn't look like a regular air bubble, it almost looks like the silicone is stretching away from the glass and leaving a vacuum space (the white streak is very thin and almost lacks dimension). I can't quite understand why a brand new professionally built tank would show this. Do you happen to build tanks?

 

[UncommonSense]

From what I can tell, this white line doesn't look like a regular air bubble, it almost looks like the silicone is stretching away from the glass and leaving a vacuum space (the white streak is very thin and almost lacks dimension).

Agreed, it looks like a void when the silicone is stretching; and if that almost-torn looking line you highlighted is the same part of the seam as image 3 in your OP, this silicone is stretching like crazy (and possibly even tearing!) when water pressure is applied…

I can't quite understand why a brand new professionally built tank would show this.

You might be amazed at some of the junk that sneaks past QC, or is moved from the QC reject pile to the “discount” pile in commercial manufacturing… i’ll dig up a pic of a big box brand tank I took apart recently that had a literal chunk of wood embedded in a bottom seam, only visible once I cut the trim off…

Do you happen to build tanks?

Kinda? I’ve technically only ever built one from scratch, but have several more in the works… most of what I do is modifying off the shelf tanks! — I by no means do this for a living…

I do however have pretty much all of 2025’s free time so far invested into an intensive self-education on all things glass and acrylic, both to find less expensive means to ends, and to improve my general skill set!

 

[RedReefer]

Is this a rimless tank because I would not use it? It appears to be separating more towards the top of the aquarium as you add more water…

 

[RedReefer]

OK, never mind I just seen the photo. I posted before it uploaded Man). I’ll probably wouldn’t use that tank. It looks like the seams are separating more water you add…

 

[BloopFish]

Is this a rimless tank because I would not use it? It appears to be separating more towards the top of the aquarium as you add more water…

Rimmed tank

 

[BloopFish]

OK, never mind I just seen the photo. I posted before it uploaded Man). I’ll probably wouldn’t use that tank. It looks like the seams are separating more water you add…

I probably won't either, but kind of curious why it does this. This is not the first brand new tank I've had this issue from multiple manufacturers.

 

[UncommonSense]

I probably won't either, but kind of curious why it does this. This is not the first brand new tank I've had this issue from multiple manufacturers.

It’s like the silicone has lost most of its structural integrity… can you say, drag a fingernail across the silicone on that interior corner and have it just scrape off? That obviously should never be possible!

I’ve never seen silicone fail in this specific way… I would be kind of curious to see the bottom seals with the bottom rim off (like the tank I referenced above)… it’s almost as if the plastic trim is doing the structural heavy lifting and the silicone’s just barely holding the water in…

 

[BloopFish]

It’s like the silicone has lost most of its structural integrity… can you say, drag a fingernail across the silicone on that interior corner and have it just scrape off? That obviously should never be possible!

I’ve never seen silicone fail in this specific way… I would be kind of curious to see the bottom seals with the bottom rim off (like the tank I referenced above)… it’s almost as if the plastic trim is doing the structural heavy lifting and the silicone’s just barely holding the water in…

It's a brand new tank and very tall. Not going to pop the bottom brace for something like that, I'd return it before going that far at that point. This tank actually has a hybrid glass and plastic bracing on the top. I wouldn't really think its the bottom panel being the issue like that picture either, as I only see this issue on one corner as well. Perhaps this tank is also just been sitting at the store for so long the silicone has aged?...

 

[UncommonSense]

It's a brand new tank and very tall. Not going to pop the bottom brace for something like that, I'd return it before going that far at that point. This tank actually has a hybrid glass and plastic bracing on the top. I wouldn't really think its the bottom panel being the issue like that picture either, as I only see this issue on one corner as well. Perhaps this tank is also just been sitting at the store for so long the silicone has aged?...

I don’t blame you there! Especially if it can be returned!

Aging was my initial thought.., but that would be at least decade or two worth!!

I’m wondering about something damaging the silicone chemically… intense, prolonged exposure to UV or concentrated ozone, perhaps? Or, direct daily sunlight exposure on an empty tank causing repeated heat cycling…. That would yield a similar effect to many years of natural aging, but only in a specific area…

 

[BloopFish]

I don’t blame you there! Especially if it can be returned!

Aging was my initial thought.., but that would be at least decade or two worth!!

I’m wondering about something damaging the silicone chemically… intense, prolonged exposure to UV or concentrated ozone, perhaps? Or, direct daily sunlight exposure on an empty tank causing repeated heat cycling…. That would yield a similar effect to many years of natural aging, but only in a specific area…

Unsure. This tank and the other tank I've had the same issue with were not in direct sunlight and were stored inside of a store for at least a year. It was definitely not in direct sunlight. I was hoping a professional builder like @cromag27 or the like was on this forum, but I think there aren't that many that are active on this forum based on the web crawling I have been doing on this subject.

 

[BloopFish]

Agreed, it looks like a void when the silicone is stretching; and if that almost-torn looking line you highlighted is the same part of the seam as image 3 in your OP, this silicone is stretching like crazy (and possibly even tearing!) when water pressure is applied…


You might be amazed at some of the junk that sneaks past QC, or is moved from the QC reject pile to the “discount” pile in commercial manufacturing… i’ll dig up a pic of a big box brand tank I took apart recently that had a literal chunk of wood embedded in a bottom seam, only visible once I cut the trim off…

Kinda? I’ve technically only ever built one from scratch, but have several more in the works… most of what I do is modifying off the shelf tanks! — I by no means do this for a living…

I do however have pretty much all of 2025’s free time so far invested into an intensive self-education on all things glass and acrylic, both to find less expensive means to ends, and to improve my general skill set!

I've resealed a tank that had a degrading inner silicone, but wasn't leaking. That was a whole chore. This tank though... the streak seems to run into the seam between the actual panels, and I don't think I have the capabilities or willpower to reseal a 30" tall tank... haha...

I believe you about the poor QC, I've been trying to pick up some larger tanks during the PetSmart sales and every single tank I've come across makes me feel very uneasy with the way the silicone is...

 

[UncommonSense]

I've resealed a tank that had a degrading inner silicone, but wasn't leaking. That was a whole chore. This tank though... the streak seems to run into the seam between the actual panels, and I don't think I have the capabilities or willpower to reseal a 30" tall tank... haha...

I believe you about the poor QC, I've been trying to pick up some larger tanks during the PetSmart sales and every single tank I've come across makes me feel very uneasy with the way the silicone is...

30” tall, 36” wide isn’t too bad; ans electric caulking gun would make short work of a reseal!

If it ends up not being returnable; you could always go that route!

Haha agreed there! I saw a 6ft long 125g (10mm thick glass) recently at my local petsmart that has the worst chip I’ve ever seen in a bottom corner of the front pane… maybe 10% of the structural silicone corner seal remained intact… they wanted 90% of MSRP for that junk!

 

[BloopFish]

I drained the tank last night. I double checked and made adjustments to make sure the tank was as level as I could make it. I started filling it up. Looked fine until approximately it was 3/8th full and I immediately started to see the "bubble" pop back up. I saw it spread real time and I grabbed my phone to show it spreading. It actually started a bit below the water level and started spreading upwards. It appears to be in the seam between where the two glass meets. I'm starting to wonder if this is water intrusion with the way it started to spread.

 

[BloopFish]

I also want to note that once it does pop up, it actually spreads above the waterline as well - approximately a few inches... so that makes me think its not water intrusion. Perhaps it is just stretching. I just don't understand why it pops up when the tank isn't even half full.

 

[gbroadbridge]

It's really not worth the gamble.

That's a lot of salt water to spread around the floor and ruin everything it touches.

If you can return it I would, otherwise maybe strip and rebuild.

 

[BloopFish]

It's really not worth the gamble.

That's a lot of salt water to spread around the floor and ruin everything it touches.

If you can return it I would, otherwise maybe strip and rebuild.

At this point I'm just wondering what is causing this though... this isn't my first tank I've had this happen to.

I'm going to try to return it. Rebuilding a 30" tall tank would be a bit too hard for me unfortunately.