I have the green algae. I have never seen black algae. I don't have a light on the tank, so algae typically doesn't grow. I read somewhere (don't know if this is true) that feeder goldfish really don't like light, which was fine with me. When our hood light rusted out, I switched the 10 gal. tank out to a glass lid and skipped the light. We recently upgraded to a tall 20 gallon, and I still don't have a top. Gotta get one soon. The glass top was more expensive than the tank. Bleh.
I suddenly got an algae burst when we hung the chick's heat lamp near it. Just about killed the feeder fish, too, because the tank heated up. They're OK now that I replaced the 250 w heat lamp with a 100 w heat lamp and lowered the lamp so it no longer shines on the fish tank.
I know my feeder fish are in the wrong size tank, so please, nobody judge me for having the wrong fish/tank ratio. I can't justify to DH to buy a larger set up. One is a comet about 5 inches long. The other is a common about 3 inches long. They were teeny, tiny little things when they were given to the kids 3 years ago. We lost one the 2nd day we had them - the common ate it's fins off. Sadly, it was the prettiest one. My oldest was devastated, so we had the big ole funeral thing. It was buried in an origami casket and we planted a magnolia tree over it. LOL. About the time the fish passed away, we had lost our dog, and my MIL, so losing that fish was just too much for the kids to take. We also put a little dinosaur ornament in the fish tank in memory of my oldest son's fish. After that, I became determined to keep the other two feeder fish alive no matter what it took. Oh goodness, what crazy things we do for our kids.
My BIL & SIL has a pond, and we're waiting for space to open up in their pond. After we do that, we'll let the kids pick new, smaller fish. When we go visit the kids won't know which fish is which. She just keeps goldfish in the pond because they have a heron problem and she restocks it each spring. Last spring I asked her if she had space, but she had just taken a bunch of goldfish from someone else, and the pond was packed full. I'm hoping she may be able to take them in the spring. But, we know if they go there, they really are going to become "feeder" fish to a heron.
Do the plants have to be planted in some sort of special dirt/sand? Right now, I just have fish tank rocks.