We had a failure in our filter and didn't notice it until it was too late. Once of our black goldfish has always been sensitive to stress, and he started to show the signed with white on the fins and gills.

By the time we did a partial water change and replaced the filter, the black goldfish started to show signed of fin rot. His front and belly fins are now very rotten and his tail is about 50% rotten. We bought medicine for fin rot and have put two doses in already.

The fish is now at the point that he tends to belly up a lot, but then correct himself. We haven't seen him eat at feeding time but we've noticed him potentially scavaging on the bottom of the tank. He seems very lethargic.

Is it too late for him? He's been like this for over 24 hours now.

Sorry your fish is suffering. People will ask you to answer as many of the questions in the white box up there ^^^ though before they are able to help you.

Additionally, if possible do you know what the ammonia/nitrite/nitrate/pH levels were in the tank before you did the water change and relaced the filter. Also do you have any idea how long the filter was out for?

STack as Lak says we need more info.

It may not be too late, but we need the other info to guide you further. Post back soon.

1. Tank size: 70 gallons
2. Filter: originally small no name, now, BioWheel 350
3. Water Change: Partial every month, full every quarter
4. Fish: Four other fancy goldfish besides the sick one. All between 2-3 inches in height
5. Medicine: Melafix by API
6. Food: Top Fin Shrimp Pellets & Color Fin Sinking Granuals

The sick goldfish has severe rot. It doesn't seem to be getting worse. He tends to float belly-up but is still breathing. He will correct himself and swim around, but it is kind of erratic. If he gets near the filter, it can be too powerful and he will get stuck. A nudge can get him moving for a while.

He seems to have a problem just staying upright.

do you have the results for your water parameters? ammonia, nitrite, nitrate, and pH?

just based on the additional information you provided, i would suggest looking into getting an additional filter because your biowheel 350 has a water turnover rate of 350gph, and you're aiming for at least 10 times the size of your tank, so you're looking at an ideal 700gph. so another biowheel 350 would do it.

also, did you remove the carbon from your filter before adding the melafix?

We don't have anything for doing water tests.

We're really just curious if we should isolate the fish? All of the others are perfectly fine.

Almost forgot... we use Easy Balance in the water when we do our water changes.

Unfortunetly isolating the fish isn't going to help him much. Sorry.

In order to help him you need to test the water. Having your own tests are very important, but in a pinch you can take a sample to the lfs and have them test it. This is free, however you have to have them write stuff down. Often they tell you "high this or that, low something but never give specifics. If you want to save the fish knowing the exact water params are crucial.

I am concerned about a couple things...........first being under filtration. A 350 is not going to filter enough water. GF are so very messy and require 10 times the filtration per hour. Meaning your filter (or filters) need to filter 700 gals an hour. This will need worked on in order to help the fish.
Next, water changes. Without knowing the chems of the tank changing water becomes like playing russian roulette. Especially when you are under filtrated. Sorry, I dont' mean to say all negatives, but I have to be honest with you.

What I can suggest is a 50% water change. Test the water and get back to us.

Oh one more thing.........doing a 100% change is often deadly to a tank's cycle. Are you familar with the nitrogen cycle of a tank?

How are things going?