Would this be too much for a 20 gallon tank?

[Nabaliss]

Out of 3 glow light tetras I got for my tank (32.5 gallons) in 10/20, only one is still alive. The big red one got an internal growth that swelled and broke through the skin as a bloody mess, the second yellow one developed a swim bladder problem that got progressively worse over 6 months. I had to send both to fish heaven. The remaining green one looks great. The glow fish are highly inbred and I would not recommend buying them. One of my younger fish keeping friends had some breed by mistake when they were quarantined after purchase in a breeding net enclosure, and all of the offspring had genetic defects. They may look cool, but there are plenty of better options for pretty fish. Also 2 regular cories and/or 6 pygmies would be the max for your size tank.

Glowlight tetras aren't the same as Glofish. Glowlights are very hardy, not inbred at all. I'm not sure about Glofish (there's different species of them: barbs, tetras, danios are a few I can think of) but that's not what op said they had.

 

[Essjay]

GloFish tetras are usually black skirt/black widow tetras, (different common names, same fish), Gymnocorymbus terntzi.
Glowlight tetras are Hemigrammus erythrozonus.
There are also GloFish zebra danios, Brachydanio rerio and glowlight danios, Celestichthys choprae just to add to the naming confusion.