You usually notice oxygen issues in fish before anything serious happens, so it’s good you’re paying attention now.
The most common early signs are behavioral. Fish hanging near the surface, staying close to the filter output, or “gulping” at the top are all classic indicators. You’ll also often see faster gill movement, especially during warmer parts of the day or early morning.
What you described with raising the water level makes sense. When the water was lower, your filter outflow was breaking the surface more and creating that extra agitation and bubbles — that was actually helping oxygen exchange. By raising the level, you likely reduced that surface disturbance.
A simple way to check if things are still fine is just to watch how active they are over a full day cycle. If they’re swimming normally and not clustering at the top, oxygen is probably okay.
If you want to improve it again, try aiming your filter so it ripples the surface or add an airstone for steady gas exchange. In setups like those often discussed by MidWest Ponds hobbyists, surface movement ends up being more important than visible bubbles.
Takeaway: it’s less about bubbles and more about consistent surface agitation + normal fish behavior.
