overstock fish

Question
QUESTION: hi i just had a quick stocking question.  at the moment my tank is a little overstocked waiting for my other 20 gallon to cycle to get out a big fish in there. i basically have 1 inch per gallon without even trying to(even though theres debates whether that is true or not). i was just wondering what techniques could i use when a tank is a tad overstocked to make it bareable.  should i lower the water level? do more changes? add in bacteria supplements? use something to detoxify nitrites?  feed every other day very little(which i already do) i basically have all those things avaliable 2 me even though the bacteria supplements and detoxify products i own i have no idea if they work but there there.  20 gallon tank with 20-25 inch of fish(estimation), biggest being a black shark which i want to remove.

ANSWER: could you please give me a list of your stock.  I need to know this to help you.  What are your ammonia, nirite and nitrate readings?  I need these also.  I would change 50% of water every week.  How long has this tank been set up?  Do not add anything to the tank other than dechlorinator.  i can help you more with more information.

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QUESTION: ok were looking at: 1 black shark, 2 tri colored, 2 bichirs, 2 butterfly groupers, 2 iridecent, 1 pleco, 1 raindobw shark, 1 ghost shrimp, 1 molly. my reading have been a steady no nitrite and no ammonia in it. its been set up i believe 2 months.  they are all pretty small except for 3 of them 2 of them i want to remove.  i want to get rid of 1 tri colored and the black shark. without them being in there they all look like they have alot of room for the time being.  i know its 2 much thats not even a question lol just wanna know what i can do to help with territory and also keeping water clean.

Answer
Ok, this has cleared up a lot for me.  Your tri-colored sharks are also known as bala sharks. these fish get huge and need to be in schools of 6 or more.  They need 100 gallon tanks.  The bichir gets over 2 ft long and needs to have a tank of 150 gallons or more.  I am not familiar with the groupers...the rainbow shark should not be kept with other "sharks" as they get territorial and aggressive.  If you end up keeping most of these fish they will not live long and will fight a lot.  To resolve some territory issues, you need to put a lot of rocks, driftwood, and plants in the tank.  To keep the water clean, change 50% of the water once a week.