What's the benefit of swimming?

My fish is not swimming with its tail fin! Not happy! Whats wrong with it?

• I inherited a 46 gallon bowed front aquarium. My friend who left us the aquarium bought a huge stock of fish: two Tiger Oscars, two Zebra cichlids, two jeweled cichlids, two spotted algae eaters, an unidentified algae eater, a male Malawi cichlid (very territorial and agressive!), a "grown" Blood Parrot, a baby Blood Parrot, a grey cichlid with bright orange dorsal/caudal fin, and a small female Malawi. When he bought all of these fish, they were babies, and could handle being in such a small tank. Now, almost a year later, they are getting so big, and I know from looking at other forums that my tank is entirely too small for all those fish. My first question is when I upgrade, what size would be good to fit all of them. My second question is why one of my zebra cichlids is not using its tail (caudel) fin to swim. It is just swimming around with his head much higher in the water than his tail, and it looks like it is working hard to swim. I do 25% water changes a couple of times a week, to keep the tank as clean as possible. I vacuum the gravel once every week, and my water parameters are "okay" (my nitrite levels are always .4 or .5, with all the fish in the tank, I cant get better). Nitrates are 3.0-4.0, hardness is 225, chlorine is 0, alkalinity is 80, pH is 6.8. I have unfortunately lost one of the jeweled cichlids from aggression issues, and my baby parrot hides all the time in some plants. I keep reading that the zebra cichlid might be swimming funny because of bladder issues, but Im not sure. Someone give me some advice. Im a noob, and I love the aquarium, and dont want to lose any more fish.

• Answer:

  Put it in a seperate tank one of the fish was very teritorial. I have had fish for a while. Put it in a tank alone or this fishy will be a snack for others. Fish are aggressive so they decide to hurt other that interfere with the. Just put it alone for a while until it gets better feed it frequently and put medi tabs in the tank to make it better!!!