Skimmer help!

^&right · Mar 11, 2026

I have a brand-new Red Sea Reefer 300 skimmer on a brand new 125g tank - no fish yet. Got the app downloaded and turned things on. Immediately the cup began overflowing profusely on factory settings. I turned the manual valve down as far as it can go, the pump is turned down to 40% and the clean out/dump valve is wide open (being returned to my sump) to keep the cup from overflowing even more than it already is. This video shows what I'm stuck with.

Sensor control, full cup activation and over skim regulation are all on.

What am I doing wrong??

Uncle99 · Mar 11, 2026

Assuming flow is a the lowest setting….
Is it sitting to low in the sump?
It’s a bit dark on my end.

^&right · Mar 11, 2026

It is sitting in the bottom of the sump, not elevated at all. Is it not supposed to be? Elevate it just to the top of the pump maybe?

Dom · Mar 11, 2026

You can control the level by adjusting the mixture of air and water. I believe I saw the adjustment knob to the right side of the skimmer.

Back off on the knob until the foam is just below the collection cup and fine tune it from there.

How deep is the water where the skimmer sits?

Euphyllia098 · Mar 11, 2026

In a new tank it is probably going to overflow because there is nothing to skim. Often they need to be "broken in" when they are new. Personally I'd just open the drain and let it overflow back into the tank for a few weeks.

curt · Mar 11, 2026

Yea there should be a mark for the max water height. Looks like the water is up to the neck. That would be to high.

slingfox · Mar 11, 2026

Look at the manual since it provides specific directions on how high the skimmer needs to sit. I have the RedSea 600 skimmer on my 140g RedSea tank and needed to buy a stand to sit the skimmer in to get the right height.

If you tank is new and has no livestock it can take the skimmer 1-2 weeks to break in.

Deathsquad · Mar 11, 2026

My experience with the Red Sea skimmers on the adjustment knob there is a red sticker half way down that says water level. You don’t want the water above that sticker I keep my water level in the middle of that sticker.
Also it took about 2 months for mine to get what they call broke in.

Once broke in I was never able to run mine over 60%. Also the skimmer has a little canister on the side where it pulls the air in. It has 2 air inlets I had to cap one off and things seemed to settle better.

^&right · Mar 12, 2026

Thank you Gentlemen. I already have the red knob turned all the way down. I suspect then I have two problems. One is it is sitting in at least 6" of water - too deep. Also new tank, no livestock, nothing for it to catch. I will elevate it in my sump and start there.

Thanks for the responses!

BryanM · Mar 12, 2026

6" doesn't sound too deep, but I can't find a concrete answer for what depth that thing should sit.

That's usually the error though, people dont realize that there's a depth it should be in... when I had one that's what was wrong and I had to buy a stand for it.

^&right · Mar 12, 2026

BryanM said:
6" doesn't sound too deep, but I can't find a concrete answer for what depth that thing should sit.

That's usually the error though, people dont realize that there's a depth it should be in... when I had one that's what was wrong and I had to buy a stand for it.

@BryanM what did you use for a stand. Been rolling that around in my head.

It's running much better today, but I still have the drain valve running open back into the sump.

BryanM · Mar 13, 2026

^&right said:
@BryanM what did you use for a stand. Been rolling that around in my head.

It's running much better today, but I still have the drain valve running open back into the sump.

I used this: