Diseases in freshwater aquarium fishes

Question
Hi Trish,
I have a 15 ltr. cylindrical aquarium in which I have kept one goldfish, one black color fish I suppose another variety of goldfish and two small freshwater fishes. My question is that lately I noticed that there is a black lining on the edges of the fins of all my fishes. I didnt notice it earlier and am little worried as to the cause of it. I regularly change water once a week. Provide food to the fishes twice a day morning and night. I have a real water plant , one artificial plant, a filter plant attached to a air pump which is on 24x7 and gravel in the fish tank. Please do advise if the black mark on the fins of my fishes is some kind of disease and how to cure it.
Thanks

Answer
Hi Aarti. Listen, it sounds like you are trying to take wonderful care of your fish, and I commend you for that.

HOWEVER, I'm sitting here scratching my head wondering if you possibly made a mistake about the size tank you have?  In my calculations, a 15 litre tank is only equal to a 3 gallon tank?  Is your tank that small with 4 or 5 fish in it?  I need for you to answer that, as it is EXTREMELY important that i have this information, especially since you are telling me you have goldfish in there.  In case you weren't aware, it is recommended by experts that ONE goldfish have at least a 20 gallon tank (30 preferably), because of how big they get, and how "messy" they are. Goldfish are actually jokingly called "pig-fish" because they produce SO MUCH waste that you need excellent filtration for them.  Anything less and your fish are undoubtedly going to pass away from lack of space and filtration.  Ammonia, nitrate and nitrite levels build up so quickly in little tanks that the fish do not stand a chance.  PLUS, goldfish need far more room in which to live as they grow to be very large when kept in optimum conditions.  I have goldfish in my pond outside that are EASILY 15" long if not bigger. I'm sure you can see the problem with a fish that big living in a 3 gallon tank.

I would STRONGLY URGE you to PLEASE do one of 2 things, or you are going to wind up losing your fish (sorry to say, but in that type of condition there is no way the can survive any long length of time). You either need to purchase at least a 20 gallon tank for the fish to put in for the time being, OR, find the fish new homes and get a fish suitable for the size tank you have.

The ONLY fish I would put in to a tank that small would be a Betta. These are the fish that they have in cups in the fish stores. They are beautiful fish and would be happy to be in an environment like you are saying your fish are in now. You would care for them the same way you are caring for your fish now.


I'm really sorry I don't have anything better to offer you at this point, but I'm being totally honest with you.  It does sound like you really care for your fish, so I'm sure you can see the problem that you are presented with.  I'm sure you will try to do the right thing.

Please let me know if you have any further questions at this point.

Good luck.