Can I keep platies, mollies, plecos, and neon tetras in a 20-gallon tank?

The Tang, Clownfish, and Chromis are marine fish requiring a saltwater aquarium while guppies are freshwater fish. These fish are not compatible as they require two completely different setups to accommodate their different environmental needs. If you are going to set up a marine aquarium - a 20 gallon aquarium is too small for the fish you have listed. A small Percula Clownfish maybe suitable in a 20 gallon aquarium temporarily, but be prepared to upgrade to a larger aquarium as it grows larger. In general, 50+ gallon aquariums are recommended to properly house marine fish as they tend to grow large and require plenty of room to swim. If you are planning to set up a freshwater aquarium, a 20 gallon setup is adequate to house guppies.

Both species are excellent "beginner" fish and quite easy to care for. Your 20 gallon tank is a good size for about 6 to 8 fish of this species. 10 would be close to overcrowding the tank. Once your tank is properly cycled, you can add your fish. They are omnivorous and will readily eat flake, pellets, freeze dried and live foods. Chemistry should be monitored regularly and kept as follows: ammonia 0ppm, nitrites 0ppm, nitrates 20ppm or less, pH about 7, water temperature between 76 F and 78 F. Provide moderate cover (hiding places). These fish are live bearers, so if you have male and female then expect fry (baby fish) at some point. If not separated, fry will likely be eaten by adults.

Yes, skunk cories reach 2 inches at adulthood. The general rule for tanks is one inch of fish per gallon of water. The cories will hang around the bottom while the guppies will cruise higher. Be sure to provide plenty of cover for the cories in case the guppies get nippy.

Adding more fish would likely cause overcrowding. This, in turn, could lead to stress, illness and aggression. You are probably at or very close to maximum capacity, especially if you have fish that are still growing.

A fish tank will need a filter, a light/hood and a heater. Make sure the filter is rated for at least twice the tank volume. The heater should be adjustable since water temperature will depend on the species of fish. Good "starter" fish include Tetras, Mollies, Danios or Tetras. A new tank should be set up and left running empty for several weeks before adding fish. A pinch of flake food can be sprinkled in daily. This "cycling" will allow a population of beneficial bacteria to colonize the tank and filter. These bacteria will break down wastes (ammonia, nitrites and nitrates) which are toxic to fish.

Platys and guppies are usually both pretty docile, but sometimes it is the behavior of the individual Fish. Make sure there are plants for hiding and I would give it a try. Thank you for using Pet Coach.

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.