That's really interesting. I know this may be silly but is it possible you just need to put more water in the tank? Typically with an AIO system when there is not enough water it will drain the back chamber first (like when you don't top off with RODI water).
A common underlying cause with "too powerful return pump" problems is that there's filtration material impeding flow. Besides a dirty filter sock, a common culprit is a filter pad or sponge in the AIO sump area. This happens a lot after adding sand to a tank because the fine particulates will accumulate in these places and seriously impede flow back to the return chamber---even though the filter material doesn't look dirty to the naked eye. Not saying this is definitely your problem, but something to investigate.
Did you ever figure this out or how to fix it? I just got one of theses tanks like 3 weeks ago, and mine is doing the same thing, I removed out all the media and it still does, this. I can get it to stop doing it if i have it near overflowing out the lip glass separating the reservoir from the main tank section. I thought it was fine, with me having to put about 2 cups of water in it every day to make up for the evaporation. Because there is no room for it reduce at all before it just sucks everything out reservoir. Then I put my fish in, I had a shrimp and 2 fish go over that lip, Two by the middle part of the reservoir and the other fish went over the intake section and sucked down into my media and died. I am really annoyed with this tank, if there is a fix or solution I would like to know. I am thinking it is a design flaw in the model after seeing your post. I think the intake slot are not long enough to allow enough water through, The filter chambers before the the pump overflow walls are to tall. I bought this tank, because it looked nice because the, fluval flex 9 gallon aoi tank and was happy with it. I figured I would just got with a bigger aoi tank. I have tried reaching out to the manufacture of the tank to see if this is normal for this model, and their website will not even submit the contact form, I don't think my local fish store is going to take the tank back either since it has been three weeks and it was their last one and it was a open box model. Any help would be grateful.
I bought a 11.3g AIO Tideland and this has been a problem with mine also. It seems like the flow is too much for the tank. I was thinking about buying a bigger one but I think the issue will still be the same. I have the heater in the last compartment after filter and the flow and evaporation caused the chamber to almost be empty by the time I get home from two day vacation. Overheating plastic smell because the outflow is empty almost.
