So the problem isn't in the power supply? It's in the actual pump itself?
There would generally need to be a path for it to trip the GFCI - this could be through the power supply Y capacitor (unusual especially with a replaced power supply) or through another path (ground probe in the tank?).
All the GFCI is doing is comparing current Line to Neutral. If it goes out any other way for more than a few milliamperes it’s going to trip. This means something is earth referenced in the water in addition to the pump.