It is certainly possible to get 2 (or more) Greenfin Spotanus fish. If you buy enough common eggs, eventually you will get another.
Spotanus fish can not be bred from other species. Whilst it is possible to breed a Greenfin Spotanus using two Spotanus species, you won't have those available to you at the beginning of the game. One of the breeding pair will always need level 2 or level 3 environment.
My top tip, though is to find different magic fish and recommend the Speckled Leaffish and then breed those to raise more money. Each can be sold for at least $20 more than the Greenfin Spotanus.
Level 2 environment will be required to keep this fish in your tank indefinitely, but there is no reason why you can't keep the breeding pairs and sell the offspring until you raise enough money to research the environment.