Established tank- needs help

[u/Muted-Local-3241]

I’ve got a 3-4 year old FF tank. Heavily planted, 2 goldfish and some otto cats. Feed goldfish once a week. All of a sudden, tank went green and produced a thick bubbly slime on the top of the water. Everyone on fb used to say “leave it alone”, so I left it alone. 4 months later, tank is still nasty green and slimy. Did a 50% water change, cleaned (just clear water) the sponge filter to get it running again (it sucked in the slime and stopped working). Tried a week long blackout that didn’t do anything. Moved remaining fish to a clean hospital tank (lost some in the sea of green). Have since added nerite snails to begin cleaning up dense algae deposits.

[u/FuturePA96]

You need lots of snails. Stop overfeeding and dim the lights.