End of line QT

cilyjr · 2026-10-03T11:19:14-0400

For those running an end of the line QT, how do you deal with slow salinity increase?

For example, I have a small end of line quarantine tank. My system does a 3 gallon a day water exchange into that tank. Today. I have been dealing with the slight salinity increase due to evaporation by simply dumping in a couple of cups of fresh water a few times a week.

Because it is a water exchange system that is outleted via gravity, it would be impossible to simply add a float valve. So I'm wondering how people deal with these slow increase of salinity on systems like this. Does anybody have any clever ideas?

EricR · 2026-10-03T11:26:24-0400

Does this mean that:
- Your AWC dumps 3 gallons/day into your QT (end of line)
- That 3 gallons/day never exits the QT other than via evaporation
...just trying to understand

*or maybe that 3 gallons/day still overflows out of QT, you just never add any fresh water to handle evaporation?
NEVER MIND -- I think that's what you mean -- can't figure out how to delete this post,,, sorry

P.S. I don't do anything like this but was just curious

ReeferTCG · 2026-10-03T11:34:45-0400

I have overflow the end of line QT so thath old SW leaves the QT that way I also have a cheap gravity fed ATO that is a bit below the overflow so at night where there is no tank water from main tank coming in and water goes down the ATO drops fresh water. QT is usually 1 ppt higher than my Display this way

cilyjr · 2026-10-03T11:35:03-0400

EricR said:
*or maybe that 3 gallons/day still overflows out of QT, you just never add any fresh water?

This.

And actually I am adding freshwater manually in the amount of about 4 cups a week. So we are talking about a pretty slow increase.

3 gallons leave the main system daily.
Enters the qt. There is a gravity drain at water level so when water enters the qt tank older water is pushed out the gravity drain.

3 gallons of water is two little an amount to deal with the slight evaporation in that aquarium. So over the course of about a week and a half I will get a slight salinity rise. We are talking 1.025 to 1.026 or 7 over the course of a week. So it's pretty slight but it still exists. I could dramatically up the amount of water that is exchanged and that would slow the salinity rise significantly, but then I would have to go through significantly more make up water

EricR · 2026-10-03T11:37:56-0400

So you need a separate AWC system on your QT that replaces saltwater with freshwater that you can dial in -- DOH!

cilyjr · 2026-10-03T12:30:43-0400

EricR said:
So you need a separate AWC system on your QT that replaces saltwater with freshwater that you can dial in -- DOH!

Okay, in your opinion how does that work since you cannot use float valve or float switch?

cilyjr · 2026-10-03T12:34:39-0400

ReeferTCG said:
I have overflow the end of line QT so thath old SW leaves the QT that way I also have a cheap gravity fed ATO that is a bit below the overflow so at night where there is no tank water from main tank coming in and water goes down the ATO drops fresh water. QT is usually 1 ppt higher than my Display this way

Okay, I see some merit to this. My issue however, is that my system does exchanges 24 hours a day. I imagine I could change that easily enough

EricR · 2026-10-03T12:35:28-0400

cilyjr said:
Okay, in your opinion how does that work since you cannot use float valve or float switch?

I was actually kinda joking and know nothing but I figured you'd use two dosing pumps to extract saltwater and add freshwater, then dial-in the amount via timer or whatever.
*not suggesting that's financially reasonable in this case though

cilyjr · 2026-10-03T13:10:26-0400

EricR said:
I was actually kinda joking and know nothing but I figured you'd use two dosing pumps to extract saltwater and add freshwater, then dial-in the amount via timer or whatever.
*not suggesting that's financially reasonable in this case though

Actually, there's some merit to this?, I could simply add a additional pump that would just pull fresh water from time to time. I would have to monitor for a while to really dial it in. That might be the easiest solution for me.

I wouldn't need 2. As the salt water change out already uses two pumps. I would just need to make an educated guess at roughly how much fresh water needed adding I may do this

EricR · 2026-10-03T13:15:21-0400

cilyjr said:
Actually, there's some merit to this?, I could simply add a additional pump that would just pull fresh water from time to time. I would have to monitor for a while to really dial it in. That might be the easiest solution for me.

Makes sense if it's worth the money.
Basically, just automate what you're doing manually now (dumping in RODI periodically).

exnisstech · 2026-10-03T13:19:38-0400

Sorry I guess I'm miss understanding. I change 4 liters 5 times a day with two pumps in and out and have no problem with an ATO keeping up with evaporation.

cilyjr · 2026-10-03T13:20:16-0400

EricR said:
Makes sense if it's worth the money.
Basically, just automate what you're doing manually now (dumping in RODI periodically).

The money won't be much compared to my overall investment. I have a large aquarium that is very automated. I have several peristaltic pumps not in use. I'll just have to set it up.

cilyjr · 2026-10-03T13:26:52-0400

exnisstech said:
Sorry I guess I'm miss understanding. I change 4 liters 5 times a day with two pumps in and out and have no problem with an ATO keeping up with evaporation.

Maybe, qt it is not large 20 gallons total. And having an apex do the change with their awc programing with out any modifications it does several very small exchanges daily. As a result and with surface agitation a float valve or switch was having a hard time. So salinity will slowly increase over time.

Hope that makes more sense

exnisstech · 2026-10-03T13:32:09-0400

cilyjr said:
Maybe, qt it is not large 20 gallons total. And having an apex do the change with their awc programing with out any modifications it does several very small exchanges daily. As a result and with surface agitation a float valve or switch was having a hard time. So salinity will slowly increase over time.

Hope that makes more sense

Gotcha. Ya it would probably have to be an ATO with an optical sensor vs a float valve. I have a cheap one ($40ish) in a 40g breeder with no sump and it works very well even with a lot of surface agitation. My RO system will to turn on if I barely bump the brute with a float valve.

cilyjr · 2026-10-03T13:53:08-0400

Thanks all for the input,
For now I think I'll try @EricR s solution of a peristaltic pump and dialing in the fresh water inlet. I know it will still creep in one direction or another but I already check it fairly regularly as is.