My 20-gallon tank has 13 fish. Can I add more without overcrowding?

Thanks for reaching out about your fish. Typically, most recommendations are 1" of fish per 3 liters of water. The better the filter system, the better the water quality and the more biomass can be handled. Remember, fish grow and therefore the biomass will increase. Good luck.

In general these fish can all be housed together safely. The one thing to keep in mind is the amount of space needed per fish to maintain a safe and healthy environment. Each 1in of fish needs 1 gallon of water. So in a 20 gallon tan you can have 20 inches of fish. The Pleco is the only one I have concerns with as some varieties can grow quite large. Check with your local pet shop as to what needs best suit this type of fish or what type of fish suit your tank best.

The rule of thumb is 1.5" of fish per gallon of tank. So estimate the length of each fish you have, add them all and divide by 1.5. This will give you the size tank you will need in gallons. In a 30 gallon tank you can keep 45 inches of fish (i,e, 45 fish, each 1 " long or 30 fish , each 1.5" long, etc. Do not forget to leave some space for growth of fish.

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.

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.

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.

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.