Just bought a 20 long from petco I ordered it offline for 26 dolars then picked it up in store I get all my filters and lights off amazon. I already had a filter but am getting a heater and light for under 50 bucks and I have all my tanks open top. But I think you can get a marineland penguin...
I would try using jungle brand fungus tablets that's what I use for my fish if they have and spots like that and it has always worked. I usually get it at walmart in the fish section. I just recently had a guppy with a spot on its side last week and it fixed it.
I could try the silicone I haven't really seen the whole crack because there is water flowing over it and some rocks stuck in thr concrete. The previous owner made it and it wasn't the best design.
I'll take a pic later today when I go over there.