I was told one rainbow shark is okay as long as I don't plan on overcrowding my tank, which I don't.
Not everyone has taken care of this particular creature, the person may be well meaning and just repeating what they were told.
If you have decided on this as a fish you want - you would need a bigger tank.
Here is a good basic resource for getting an idea of stocking your tank. You put in the type of fish and it helps give you some suggestions.
AqAdvisor - Intelligent Freshwater Tropical Fish Aquarium Stocking Calculator and Aquarium Tank/Filter Advisor
There are numerous reasons to follow suggested guidelines for tank sizes.
Some suggestions are able to be bent, by doing additional pwc, meticulous husbandry, lots of additional filtration capacity, careful feeding, etc. But you will never be able to give a fish like that the proper space for growing up in a 20G and he will be stunted which is unhealthy for the fish, and is known to lead to a shorter lifespan and tendency for illness.
To be honest MOST fish are not kept with that required attention to detail for best practices, and bending the rules.
Most people here just want to see the fish have the best life possible.
When fish are kept well; good conditions, feeding, environment they thrive in, tankmates they can coexist with, etc. - you will have less struggle with disease, fighting - they will be healthy and strong - less stress and a happier experience for you AND the fish. Win, win!
I am glad you are checking in to finding out what the fish for your tank need to provide you with an enjoyable experience in fish keeping and them a good responsible home.
Fish keeping is exciting and there are so many awesome choices of fish available.
You have the power to make decisions for your tank and fish keeping experience. Learning and research helps fish enthusiasts make the best decisions for a tank and stocking, care and feeding, etc.
By not paying attention to /understanding the size tank it needs, you will have even more aggression due to the shark not being able to do what it would naturally which is cruise around it's territory and keep unwanted fish away. All the fish in a 20G tank would be in his territory and he would want to keep them away, all would be in danger of harrassment.
An easy solution is to PLAN a tank size FOR the type of fish you want to keep. Get a bigger tank if you want to keep a RTS. Select a different type of fish if you don't want to get a bigger tank. There are so many choices. The RTS will not be able to live an enjoyable life or long life in a 20G.
Some DW gives off tannins coloring the water for months. As mentioned pwc or chemical ie Purigen or carbon. Purigen makes my water look great in the tanks it is in. Some people do not want the chemical reaction of Purigen binding things in the water.