On the 40, i have a eheim classic canister for a 60 gal and a penguin sponge filter in the back.
The loaches are good, obviously i would like to have them in a bigger tank right now. I dont believe they have become stunted yet, iv'e heard there is or can be health ramifications to having them in the smaller tank. Though in my experince, we had a similar 40 gallon with a loach pair that lived to be almost 40 when i was a child, growing no longer than 5 inches. Of course that is not ideal and i plan on moving them in the near future

. But for now they are happy and healthy.