One question that keeps coming up is what fish will survive in Level 1 or Level 2 environments. So I decided to do a bit of research. (as I progressed, I bought the aerator, temperature regulator and snail, as well as plants of nutrition and curing. I also fully researched the food upgrades. (didn't bother with any of the ornaments or advertising research...)

First, here are the fin and body types that show up in common eggs:

Greenfin, Orange, Pink, Silky, Spined, and Twinfin
Beta, Fatfish, Fruitfish, Shark, Spotanus, and Stickfish

Any fish with one of the above fins and bodies (36 breeds) will survive without extra vitamins in all levels.

I made a matrix were I bred the above fin and body types and the following set is the "second generation" fish fin and body types:

Flagged, Golden, Hornet, Speckled, and Wasp
Arrowfish, Bananafish, Goldbulb, Goldshark, and Leaffish

Any fish with any of the 11 fin or 11 body types (121 breeds) will live to adulthood in all levels.

Fish that have at least one 2nd generation attribute (fin or body) will need vitamins to retain health in Level 1 environment.

An interesting fact is that cross-breeding two first generation fins or bodies or two second generation fins or bodies will produce other first or second generation fins or bodies and those will survive.

i.e. a silky goldbulb with a twinfin leaffish will produce a golden stickfish.

HOWEVER, cross-breeding a first and second generation fin or a first and second generation body will produce a hybrid to fragile to survive past age 5 in Level 1 or to adulthood in Level 2.

There may be a few exceptions to this, I've twice had a third generation fin type make it to adult, but died before I had a chance to see if vitamins would prolong their life... I may have also had a non-surviving Orange Snooper help prolong things but didn't make it to adulthood.

While in Environment Level 2, I also decided to see what sort of fish got produced from the Unusual and Rare eggs.

I was not too surprised that the Unusual eggs produced fish that were 1st or 2nd generation fins or bodies. They need more care than common 1st generation fish.

However, I was disappointed when the Rare eggs also seemed to only produce fish that were 1st or 2nd generation fins or bodies. There doesn't seem to be much reason to buy the rare eggs when it is as likely to get the same fish from the unusual eggs. I was hoping for "3rd" generation fish, those that don't survive to adulthood in a Level 2 environment.

I hope all of this helps someone.