Crayfish or Goldfish?

I would get the goldfish -- just because I like goldfish!

Edit: To expand further, 20 gallons is, according to most websites dedicated to fishkeeping and goldfish keeping in particular, the minimum for a single fancy goldfish -- with double filtration which you have covered. Opinions to differ here, and across sites, but that is the most common one I have encountered.

I like the goldfish because they are cooler to look at and yes this tank is big enough for it! Hate when ppl say it isn't. Goldfish are easier to take care of beucase of the temperature and things so if that's what you want ya know that's good haha.

First a crayfish has to be alone in a fish tank and any kind of pump is better but if you go for a goldfish you can get 1-3 in a 20gallon fish tank i had 4 and they were bigger then they were supposed to be

20 gallons for a goldfish, no. You will need to be bigger one in a few months. Go with the crayfish. They are cooler. Goldfish are pigs with fins.