I've painted a couple of fibreglass or composite tanks and have always used a good quality filler primer the same as what auto painters use on car bumbers.
Go to a local auto paint supplier and tell them what you are going to paint and they will advise on the best product to buy.
As bullylover says, ensure your prep is 100% and dust free because it will show on the final coats. Use fine grit sand paper to flat the primer coats smooth.
Good luck.