At the moment, you're overstocked and have fish that aren't suited to your size tank
The Dojo and Zebra Loach are soft water fish, and the Dojo loach also gets quite large. They are also cooler water fish, prefering water colder than the other fish in your tank really like.
There's also the problem of the rainbow fish, they really do best in larger groups (thinking around 5 at the minimum) and are active swimmers. Most people recommend keeping them in a 55 gallon or larger, based on their active nature and their adult size.
It's a similar situation for your danios...these are active, shoaling fish. While they may have enough room in the 20 (not sure exactly on that), they do prefer larger groups (thinking 6 minimum here).
The zebra loach is also a shoaling fish.
Without proper schools and tank sizes, these fish will be under stress and more likely to have health issues and not live as long as they should. There is also the possibility of aggression resulting from being kept in improper numbers.
Personally, I recommend rehoming the majority of your fish, and getting some more suited to your tank size, such as male guppies (very colorful, fun fish that come in a whole slew of varieties).
Also, what type of pleco do you have? Many will quickly outgrow a 20 gallon tank, and with such a high bioload they can really limit the amount of other fish you can keep.
forum community dedicated to tropical fish owners and enthusiasts. Come join the discussion about species,breeding, health, behavior, aquariums, adopting, care, classifieds, and more! Open to fish, plants and reptiles living in freshwater or saltwater environments.