You need to try and locate the source before you can truly address the issue. Tiny bubbles get released into the water generally from an intake (whether that be from a filter or powerhead) when it "sucks air" meaning that there is a breach in suction. This can be common in venturi systems, as well as skimmers. Generally speaking, powerheads won't push out micro-bubbles as their turn over is exceptionally fast and the units are completely submerged and posses single stage distributions (Water being sucked in, and blown out an opposite end from a propeller). It has no chambers or media to travel through, so when bubbles appear, it's rare, and they are larger.
Checking the seals on your units would be the best place to start, and if you are able to locate the source of the bubbles (meaning that you identify which particular unit is creating them), then you can start a process of elimination to rectify the situation. I'd start with making sure that your seals are tight, your propellers are in good condition and free of build up, and that your unit is performing as expected, meaning that the suction of the intake as well as the return flow is steady and strong. Breaks in the return flow can indicate dirty filters, weakened suction, as well as diminished performance of internal mechanical pieces to a unit. a lot of times, finding replacement parts for units isn't all that difficult, you just need to identify the issue first.
What types of circulation and filtration are you using (Brands and Models)?
You'd want to find a solution soon, because if these bubbles are as tiny as you claim, and filling the tank, you could soon find yourself dealing with a more serious situation - super saturation.