We have had 2 goldfish for just over 18 months.
They seem happy and have grown well. They have a tank that is the right size for them and I cycle the water every 2 weeks. I change about 1/3 - 1/4 of the water.
This morning we came down to find one of them looks like he's about to die. He is primarily on the bottom of the tank and on his side.
Nothing in their environment/feeding etc has changed. The only think I can think of is that about 2 weeks ago I noticed the sides of the tank were a bit grim with algae type stuff. I've never had this before.
Any tips to revive him? Or to prevent the other one going the same way?