My freshwater Fishes are dying

Question
i recently put a 'suckermouth' in my tank and the next morning all the other fish were lying on the bottom of the tank. so i 've changed  the water two times now and isolated dat suckermouth. And added 'Aquarium conditioner' and 'Fish care - blue', since then some are better and others tend to drift to bottom of the tank. also moslty they hang upside down. there are 14 fish in total, Tank size is 3'x2.5'and it has a bed filter, and a normal filter. The fishes that are still unhealthy is called 'Koi'. And i have 6 of them. i also hav 2 anglerfish but isolated them after i saw them biting other fish. So it Would very kind of u if u could help. please send me what to do to my e-mail address (shah009@hotmail.com)

thank you

Answer
Dimelo,,                                                     Unfortunately it only takes one fish to crash your tank. I think the pleco was the one. You know that your koi are beautiful, but extremely dirty fish . That last fish probably put your ammonia levels to the top. Your fish probably had acclimated to the high ammonia conditions and that one put it over the edge. I am glad you took him out.You did syphon the rocks when you changed your water I hope. The drastic water changes probably didn't help if they are used to a regular water change so many times a month or week. The only thing I can suggest to you is check your ammonia levels. Check your nitrites and nitrates too. Try not to do anything more drastic right now. Give them some frozen peas squeezed between your fingers to prevent swimbladder infection after you medicate with what I am about to tell you.I think I would add some tetracycline. You don't know what the sucker had. Swimming not upright is a sign of swimbladder infection , which is bacterial. Tetracycline is a broad spectrum antibiotic.Don't forget to take out your carbon for the 7 days you medicate. Also after this is all over get a veggie clip from the pet shop and slice orange into not too thin slices and cut the rind off half. Great for vitamin c . Your goldies(koi) will love you for it. Treat your sucker too. You have alot of koi in a small tank. As they grow I think you are going to have a problem keeping them in that tank. Did you bring them in from the pond for the winter. If your pond is at least 3 foot deep they will hibernate not necessary to bring them in. If you are keeping them as pets get a bigger tank. For that many I would go as big as you have room for. Like maybe 125 gallons. These fish are going to grow. Let me know what is going on . Good Luck, Tina