Welcome to AC!
Will you start with young angels & wait for them to pair up? 6 is the "usual" number for that so you'll get at least 1 compatible pair in a 4ft tank. For an actual pair a 29g is big enough. If you think you can breed them in a 4ft tank with other fish, you can. But the pair will defend a 3ft area & all other fish will be forced into a 1 ft space...with forays to eat eggs or fry. My last spawners was careful to spit the fry into the filter intake...I should have used a sponge over the intake but I didn't really want fry then. Still, it was had to watch...
Buying an adult "proven" pair is both more expensive & you may get old fish that are past their prime breeding years...There's a reason they're available...
As for beneficial to the hobby...Do you mean most sellable or for the highest price? You'll need to research, a few years ago it was "blue" angels. Someone in my club is working on non striped blues by selective breeding, a long term project. Blues are still expensive!!
Koi angels are pretty & pretty common. You might ask or at least look around at shops near you to see if they sell locally breed angel strains...& pick a different 1. Or maybe they say we'll take all koi, zebras or blacks you can breed (good luck!)...some offer cash, some just store credit...& you'll need to know at what size they will buy fry too. Feeding a tankful of fry can get expensive & a lot of work keeping the water very, very clean...& angels can breed every 30 days...or sometimes more! You'll need a place for the pair or the current batch of fry (hint: adults are easier catch). To stop the breeding madness you'll have to separate the pair...with
And then there's culling, a nice word for killing off the less than perfect fry. Missing or deformed fins or maybe some other issue, failure to thrive (grow well). This was always hard for me...it helps to have some fish that will eat them at any size...The more heavily line bred, the more culls there may be...
Still, with all that, it's fun to breed fish. I especially like the way cichlid parents care for the eggs & fry...for a while. I hope this is not to be much a money maker for you, in reality not so much...not a retirement plan by a long stretch, lol. Just some stuff to think about.
