Yeah the 15G tank is a fry dedicated tank they wont need much protection from other fry wanted to put the hoplos in the main 55G tank or would that be to small also? :0? I'll make sure I get a male then.
55G is pretty ideal for hoplo (Megalechis Thoracata).
Best if it's 4 foot long, can deal with 3 foot too (mine's 20 gallon 3 foot long).
But be careful as some will also have this odd habit of taking pieces of plants to make a bubble nest (that's what mine does when he sees fry and he gets a cool electric blue color on his fins and belly, but he fails to make a proper nest and I just end up with a bunch of plants stuffed somewhere in a corner or under my fry cage).
They grow to 13-15cm long, sometimes 17cm.
Planet Catfish on Megalechis Thoracata
They are also greedy when it comes to food, I feed mine bloodworms, flakes and tiny pieces of salad. They will push away other fish from the food and steal food from snails.
Also, I think you should only have one male hoplo as they might not get along with other males (mine did when I had 2 males, but that may be an exception). And they are pretty brutal when defending fry, today I saw one of the fry get eaten by a lyretail swordie and the hoplo rammed her into the glass to move her away.
I don't quite understand what's up with mine though, as it has these things on the pectoral fins, while other hoplos I've had didn't have them. Those things that point towards his back, not the hard edges (which tell that it's a male).
It is hard to tell which is a male while they're young though, you'd have to look under the fish to see if they have a white thing dangling from between their fins.
As far as I know, that thing's there even when they're young, but the pectorals are not evident before the age of 1 year.