I had painted my 75 gallon black about a month ago. I bought the cheap acrylics from Wal-Mart. I think I paid $2 for it. . .
It went on really nice. Took about 3 coats to get it nice and covered. I was happy.
UNTIL. . .
I was fiddling with the intake about a week after painting and I managed to get a few water drips down the back of the tank. Well, I look in my tank and what do I see? Water stains running down the back! Lucky me!
I tried drying them with a hairdryer. . .no luck.
I tried painting over them. . .no luck.
So, I ended up moving the (full!) tank away from the wall (so glad I put the felt pads under the stand otherwise there is no way that sucker would have slid out). I scraped the old paint off (it came off really nice and only took about 5 minutes with a straight razor)
Then I ran out to Home Depot and bought a small tub (they call it the "test size") of latex based paint. Had them tint it black (which is free of course) and re-rolled that on. It looks so much nicer than the cheap stuff (while still being cheap itself. ..I think it was $4.99 for the test size). And the water doesn't effect it at all.
Just my opinion, but next time around I will avoid the cheap acrylics and spend a few extra bucks for a decent product. Maybe it was just a REALLY cheap brand of acrylic. . .but yeah, water stains running down the back didn't look very nice!
Good luck with the painting.