That shouldn't be too many fish biol-load for a 55 gallon tank, one that has been through the Nitrogen Cycle. What kind of filter / medium are you using and how long has it been going? Water changes are like bandaids / quick fixes, but they don't usually get to the root cause of the problem.

Ammonia comes from the waste products of the fish and if the Nitrosamonas & Nitrobacter bacteria haven't had time enough to establish themselves, then the bio-load is overwhelming. Obviously, adding new fish won't help. Do you have another filter, one that has been "seasoned"?

Changing the water in those percentages is doing a lot more damage than you think, the ammonia is not going to go away just like that, adding chemicals also is not going to help, you can pour a gallon of Ammo Lock and, yes, the ammonia will rise again, so:

1.- Stop changing the water, you are doing more bad than good.

2.- Check your filter media, clogging of the filter media causes levels to rise.

3.- Do not add chemicals to the water, they only worm for a very brief period of time ( hours ).

4.- If you have an external filter substitute the activated charcoal for zeolite, let it run for 24 hours and then rinse the zeolite in saltwater ( 32 grams of rock salt to a quart of water ) to reactivate the zeolite, rinse in fresh water and insert it again, zeolite is good for 3 times and then it must replaced with fresh zeolite, that will keep the ammonia levels "manageable" until the bacteria can deal with the ammonia.

5.- Increase the aereation, nitrifying bacteria need oxygen and so do fish, ammonia causes swelling of the gill lamelae and fish can´t breathe.

6.- As ammonia levels drop begin to monitor nitrite levels, the increased aereation must continue until the nitrite levels have dropped to zero. Nitrites transform hemogoblin into metahemoglobin and the gas interchange in the red blood cells can´t take place.

I would test your tap water to verify the kit is working. Water changes should eventually get the ammo down if its not building faster than your removing it [overfeeding?] 4.0 seems way to high to have any fish survive. Bring a water sample to fish store to verify your findings. Also check ph and nitrites. The tank may be cycleing. Like Raul said forget the chemicals they tend to mask the problems even if they appear to be working. Your filter may not be working properly [clogged or water bypassing the filter]. Add a second filter, it's cheap insurance ans lessens the ammo spike when changing filter material. With patience you'll get through this. What was your maintenance routien before you noticed the problem? Regular water and filter changes?

First off let me assure you that as long as you are neutralizing the chloramines,and the new water is the same temp as your tank you can change 100% an hour and do no harm.The fallacy about water changes being bad comes from tanks where changes are neglected long-term.In that case a large water change can alter parameters enough to induce shock.You should get into the habit of changing 25%-33% weekly as a general rule.But you have an immediate problem that needs addressing.Zeolite is a short term fix to ammonia problems.You need permanent biological filtration.What brand and model of power filter do you have?They are not all equal and the best way to maintain them varies from brand to brand.More info would make it easier to give you specific recommendations.As far as feeding goes,you need to be careful.Food should NEVER lay about uneaten more than a minute or two.Follow this ruleand feed once a day and you can be sure that overfeeding is not the problem.The most important thing to do is to stop adding fish until you can maintain a stable population for at least a couple of weeks.Do not EVER run out to replace fish that died!Post back if you need more info.Good luck!

One of the new fish could have been diseased and spread it to the rest of the fish. Happened to me, wiped out my shark and angel fish.

My goldfish started to get sick with ich and I check the PH and it was to high so I checked ammonia 3.0 the nitrate 0, nitrite 0, PH 7.2. Tap water .50 for ammonia PH tap 0, Nitrate tap 0, Nitrite 0. I have removed my fish 4 goldfish small to medium size and started treament for the ich they are in a bucket with an air pump. I just check the water again and the ammonia is between 1.5 and 2.0. I did a 80% water change and it is not doing the trick. I have a 39 gal Oceanic self contained the filter is a Fluval 205 which I was told is a good filteration system and can handle the tank size. Have you any answers for this problem

Use EcoBio-Block! It works wonders. I love this product. It keeps my tank nice and clear, and I don't have any problems with ammonia.

I also have a 55 gallon. Depends, of course, on your long term goals, but I think really you have too many fish. A pair of angel fish would be about right in a 55, when full grown. The Plec has to be a baby because they get monstrously huge as they mature. I bred gourmai back in the 80's. They love green water. The tetras generally prefer soft clear water, the danios more neutral.

Do a water change. Go to a garden supply and buy a small bag of white rocks, add a few to your tank. They will help buffer the water. If your tank has gravel or an undergravel filter system, get rid of them as they easily become filthy and add to the ammonia problem. A better method is to have porcelain tile cut to fit the bottom of your tank. If you are growing live plants pot them and light them as you would a house plant.

Overall, those white rocks I mentioned might do the short term trick for you, because it sounds like your tank has no buffering capacity.

Check your gravel because I also am having problems with ammonia levels and noticed a blue dye from my new blue lagoon rock and all so check your filter settings to make sure it is drawing water and fish waste