well crowntails dont really require any different care than your other bettas, nor are they any less/more hardy. however imo it can be easier to see their health becuase of their tails. since you mentioned that their tails looked stringy, that usually happens becuase of water quality. my CT had stringy tail spikes until he was out of the petstore water. once you get your CT settled in a good tank then they usually improve.. just like any other betta. their tails arent always stringy looking, its becuase of the water quality. tho sometimes, it may be because they are really young. their tails fill out more as they grow..
dont let their stringy tails hold you back! they will look better once you settle them into your tank! :D