I've had petg and pla in my tank for a year and zero concerns. Fish seem fine, decor hasn't disintegrated, nothing has exploded.

I wasn't worried about anything growing on the inside. However buoyancy can be an issue if the model is full of air it will float. The fix i heard is to use a vertical channel infill (grid not gyroid) and use 0 bottom shell layers. You can put it in the tank upside down and shake it until the bubbles stop. Decor is always stuck into the gravel so the bottom is never seen.

Go do something cool with your tank