Can I put marine fish and guppies together in a 20-gallon tank?

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.

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.

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.

Rainbow sharks are generally aggressive with other fish and are not a good match with peaceful fish. Tetras and guppies are generally fine to live together. When picking out fish, you want to keep a community that all have the same aggression levels. The store clerk will be able to help you with that information. I hope this is helpful and please feel free to post any additional questions.

Thanks for reaching out, the rule of thumb is 1" of fish per 3 gallons, but smaller tanks are very difficult to keep stable, and saltwater fish are easily stressed by environmental fluctuations (salinity, temperature, dissolved oxygen). Good luck.