My fish are too big for my tanks. What size aquarium do I need?

A 55 gallon tank is truly not large enough for an Oscar. Usually they are housed singly or in a mated pair. A single adult Oscar should have around a 75 gallon tank MINIMUM and a pair needs 100G or larger. Here's a bit more info on Oscars https://www.petcoach.co/article/oscar/

The goldfish can get very big and may require a significantly larger tank as they grow and grow. As a rule of thumb, you need 1 gallon of water per inch of body length of fish. So if you have three 1 inch fish, you will need 3 gallons of water. I suspect you are at your max already in the smaller tank. Just so you know, most people do not have goldfish in tanks with tropical fish. You may consider getting two tanks One for your cold water fish like the goldfish, and one for your tropical fish that require a warm water temperature. For the goldfish I would suggest a minimum of a 20 gallon tank. For the Danio, I suggest to get a few more of these as they are schooling fish and a 10 gallon would be just fine. I hope this is helpful and please feel free to post any additional questions.

Thank you for using Petco Pet Education Center, formerly Petcoach! It looks like the tank provides enough room for Speedy and Explorer to swim comfortably. Keep an eye on them, and move them into a larger tank if they get significantly larger or you notice signs of fighting or increasing ammonia levels in the tank.

As long as thee oscars are small there will be no problem, however they grow very fast and they will eat all your other fish except the gouramis. So yes, you will need a separate tank for them and preferably at least a 40 gallon tank. The same is true for the parrots, another cichlid species.

I would strongly recommend against keeping these 4 fish in one tank. Oscars can reach 12 inches when fully grown. JDs can reach 8-10 inches. You will run out of tank space eventually, or fish if they, being cichlids, become aggressive.

Check the water chemistry and temperature. Use a home test kit or submit a water sample to test fro elevated ammonia or nitrite levels. Most pet stores will do this for free or for a small fee. Partial water changes can be done, if indicated, to stabilize chemistry and improve or maintain water quality. Ideally, you want 0 ppm ammonia and nitrites with nitrates at 20 ppm or less in a non-planted tank or 40 ppm or less in a planted tank. Use only aged or conditioned water that is temperature matched to prevent stress or shock. You'll need to keep the temp at the upper limits for Red Tails in order to keep the Angelfish comfortable. That would be in the 78F to 80 F range. Both have similar pH requirements, 6.8 to 7.5. You can fast the Angel for 2-3 days then feed bits of cooked, shelled green peas. This will usually resolve constipation and bloating. Pristine water quality may resolve the patch on your shark. Recommending a medication without more information or a clear image would be risky to the stability of the tank's nitrogen cycle. You can use plain aquarium salt (NOT table or Epsom salt) as a general tonic. It will reduce stress, improve gill function, may treat minor infection and aid in recovery. Dissolve 1 tablespoon per 5 gallons of water in a cup of tank water and gently pour it into the tank. Treat for 10-14 days then reassess the condition. A note on tank size: Although the two fish may be fine in a 15 gallon tank now, you will likely need a larger tank as they mature. Red Tails can reach 6-7 inches and Angels can reach 6-8 inches. At adult size, a 15 gallon tank will be insufficient and could lead to some aggression from either fish.

As for what kind of tank you will need: Basically a square fish tank, do not get a bowl. You will likely need a heater and an air pump, and a thermometer, It is easiest to by a complete kit at a pet shop. As for size of tank: The bigger is always the better....however a minimum size will be determined by the type of fish and their average size they will grow into, So once we know what type of fish you have and how many, we can tell you how big your tank needs to be and what type of "furniture" (rocks, plants, gravel) you will need.