how much water are you filtering?
how many water dragons
probably an internal filter with sponge in it would be easiest but will need cleaning often
or a large capacity canister
do you have pic of setup?
don't forget your water dragons defecate in the water
so you will need a good filtration system or very frequent water changes
i ran swordtails for a long time with wd's, they bred well
giving you a species name is hard, but a colour name might be easier
try here for some photos for reference
http://www.fishlore.com/fishforum/angel ... lfish.html