The issue is the genetic 'cleanliness' of the animals. Some zebrafish will only glow red when you feed them special stuff, some glow green, some only glow green in certain parts of their yolks/chorion. Sometimes this only happens with every other generation of females.

It's very complicated and quite 'messy' when it comes to the phylogeny/genetic trees. Most of the time, researchers only need those 20 fish or so once every few years, if that. Sometimes they need to interbreed these different fish to glow green and red. There are literally tens of thousands of types of fish that have to be kept. As an example: Jackson Labs has mice ready to order to the specific genotype that you want and it gets very specific [0]; zebrafish aren't quite at that level yet, but are getting there.

But the effort is worth it. Zebrafish are a great animal for studying neuro development and many other things. Many of the proposed treatments for ALS, Alzheimer's, Parkinson's, etc. got their start in zebrafish and in studying how those little vertebrates function. The effort will save countless human lives.

[0]https://www.jax.org/strain/008199