My goldfish has black on the edges of his fins; what is this?

1. Your tank is very overstocked.

Two fancy goldfish require an absolute bare minimum of 30 gallons with double the filtration. A single fancy goldfish requires no less than 20 gallons. Do not add anymore fish.

2. An algae eater produces a massive amount of waste and is not compatible with goldfish. They have been known to attack goldfish in an attempt to suck off their slimecoat, and they are tropical while goldfish are coldwater fish.

- Odds are the water quality was poor already so the algae eaters died because they could not adjust.

- The temperature was probably too cold.

3. It sounds like you weren't cleaning the tank often enough to begin with and then overcleaned the aquarium. Now the goldfish is suffering from ammonia poisoning which explains the ammonia burns on the edges of his fins.

- Because the tank is overstocked you should be cleaning out about 50% of the water every week.

- Never clean the entire tank and clean out the filter at once.

EDIT- Because you introduced another fish into the tank (another fish that produced a lot of waste) that made the water quality even worse for all the fish. That's why you started to see problems.

It could be natural coloration change or it could be ammonia burns from being housed in too small of a tank. You may not have seen the problems directly, but your tank still had problems when it was just the 2 fancy goldfish. A 10 gallon tank can not handle the waste load of 2 fancy goldfish. Those fish should grow to be 6-8 inches long, much too large to even move in a 10 gallon. Keeping them in a 10 gallon their whole life will just stunt their growth and result in a slow painful death. You seemingly had no issues until you overloaded the tank with waste from the two goldfish and the algae eater.

Ammonia burn, they have been over stocked. Comets and shubunkins want a pond no longer a tank. in case you save them in a tank it must be seventy 5 gallons and 50 gallons for each extra no much less. Any smaller is merciless as they are meant to enhance a million foot in length. If stored in a small tank or bowl they'd have stunted develop the place their insides proceed to enhance interior them slowly and painfully crushing them to death. in addition they swim in a pool of their own waste. Goldfish are meant to stay atleast 15 years as much as 40 years no longer each week or perhaps 6 years. I see out of your different question if this became your question you have a small tank on your goldfish that would basically greater healthful a betta in any know. i wish you purchased the ideal length tank earlier you think of of including anymore. I tank goldfish care heavily and so could anybody else they are residing creatures and intensely smart fish.

You shouldn't have goldfish in a 10 gallon tank. They make a huge amount of waste, which will eventually poison them if the tank is too small, the filter is weak or the water hasn't been cycled properly. What you're describing sounds like ammonia burns. Did you use a water conditioner when you changed the water?

Keep your fingers crossed for the surviving fishy, but start shopping for a bigger tank.

Its fin rot again from poor water quality. The first problem is that your tank is too small for any goldfish. Second you did not cycle it.

http://www.firsttankguide.net/cycle.php

http://www.theaquariumwiki.com/Carassius_auratus