Welcome to AA!
goldfish in a 10g is a BAD idea. Goldfish are very messy and grow very large. Even a feeder goldfish will reach 12"+ in length. Keeping goldfish in a small tank will stunt their growth and most likely kill them. I would try to rehome the goldfish immediately.
Replacing your water (Doing a partial water change (PWC
)) will not get rid of many bacteria. The good nitrifying bacteria live on surfaces in your filter and tank. PWCs are a good thing.
Your water is cloudy and yellow because the fish are swimming in a high concentration of their own urine. If you do a PWC
daily or twice a day, your water should start clearing up. Getting rid of the goldfish will cure your problem.
Good job on limiting the direct sunlight. You're having algae problems because you have a lot of fertilizer (nitrates from fish waste) in the tank. More PWCs would help. Otocinclus are good algae eaters for a small tank, but they'd die from the poor water quality.
Plants are very good for fish to hide in, but many plants commonly sold aren't fully aquatic or they need fairly intense lighting. If you're set on live plants, start with anacharis. It's a low-light plant that grows nicely.
I'd hold off on trying a puffer tank until you've had some more experience. Puffers are aggressive little beasts that really need brackish water to do well. Get your 10g established and running well.
Nice score on the 55g. Off the top of my head, I'd say the glass is thick enough to be an aquarium. Generally terrariums are shorter and wider. Take a look at the silicone seams on the inside of the tank. If they're not shrunken, shriveled, or yellowed, I'd test fill the aquarium and keep a close eye on it. If the silicone isn't in great shape, take a razorblade and remove all the silicone inside the tank. Reseal it with aquarium-safe silicone. Look for clear 100% silicone without any fungicides. I believe GE-I is a commonly used brand. The stuff at your local fish stores (LFS
) is hideously expensive. Go to Home Depot, etc.
Sorry for a bunch of negativity in response to your first post. Don't get discouraged. It's a common misconception that goldfish can be kept in small tanks. Keep at it and you'll find your way.