Trouble in the Fish Tank

I am in the middle of doing a half cleaning of the aquarium.

Why? Well, I just went to give the fish dinner and saw my two algae eaters lying upside down at the bottom of the tank.

I am a little sad. I really liked them. They were so funny, always sucking on the glass.

I am also disappointed that I have lost 3 in three months now.

I don't know what it is. I think there is just enough algae, I give them vitamins, I do not have fish that are picking on them, so why are they always dying?

Well, I know what happened to the first one, he thought it would be a good idea to eat out of the filter. But the other two?

I am confused.