Recluctant Goldie breeder

[Finnruby]

Greetings! Our first, a Common from a fair 3 years ago has a beautiful Lionhead boyfriend given to us about 9 months ago. They were in a Hex 5 and began breeding non-stop about 4 months ago. I was letting nature take its course by allowing them to hatch and be devoured until my 8 year old begged me to save a batch. This led me to move the busy couple to a 10 gallon tank and removed the last spawning to the hex 5 after a week in a little portable thing. We had 16 hatch and have only lost two thus far. We just put 4 more ready to hatch eggs with them today. Three have bent tails and are erratic swimmers all of a sudden so I'm worried. What's happening? Genetics? Disease?My formerly callous attitude has now transformed to deep concern and love for our babies!

 

[Scottyhorse]

My guess is from bad water quality. Common goldfish need 75 gal+ fancies 20 gal+