I’m setting up a 180G frontosa colony with 15 frontosa and 10 clown loaches.
would 3 sponge filters be enough to keep the tank working well or will I need a couple of FX6?
Sponge filters are fine. I mix box filters (lots of air and frequently-changed fluff over bio-media) and sponges (low flow) to address mechanical + bio-filtration. Better yet would be a Poret wall and air-driven jet-lifters.
Not sure I'd mix Fronts and clown loaches, though, as the Fronts will try to eat the clowns and could die trying.