Hey Salamander
Conchs are herbivores with a voracious appetite.
Tank sizes and conditions vary. Perhaps your system is producing enough algae to provide for a fighting conch over the short term. The real question is whether or not it will sustain them over the long term. If not, you may end up having to feed them, as I ended up having to do with larger species.
Perhaps someone else may want to comment as I have not kept a fighting conch. But I strongly suspect their care is no different.
Consider keeping a smaller species. Just add a few and see how well they manage things. If over a short period of time you discover there is still too much algae, just add a few more until you find the right balance.
The downside. Smaller species may be harder to obtain. They are susceptable to prey from crabs. Their lifespan may not be as long. That said, after getting the peskly little crabs out of my system, I have several small guys going strong for well over a year now.
Cheers,
Bob