Its gonna be a tad diffuclt to paint it while there are fish in it, but people have done it before. Just be sure to tape up every thing and put something down on the floor as it may drip. You will also want to make sure the top is covered very well, the fumes can be dangerous to the fish.
I have used black krylon spray paint as well as semigloss black. With the regular paint I had to prime the glass first or it peeled off.
If you can get behind the tank all the way I dont see why spray paint wouldnt be doable. Just do it quick and make sure the coat is even. It can drip too so tape off using it as well.
180g- Mostly BIG fish and some coral. ~80g Nuvo- My coral tank with "happy fish"~ 90g- FOWLR Not the not happy type of fish~ 125g- Freshwater Malawi Cichlids ~10g- Nuvo- The refugees from the Ich of '18