At first approximation, I'd not think the fish species matter. What can matter is that whole fish is the best with head, bones, guts, scales - a more or less total nutrition package.
One could try and find some fish species closest to what gulpers usually prey on in the wild but I think this'd be an overkill.
If mostly nutrition-poor fillets are offered, than a soak in Vitachem would be desirable, or high quality pellets could be stuffed in the fillets.
