I wouldn't trust the test strips as they tend to be inaccurate. You'd be better off getting a liquid test kit which tests for ammonia, nitrite and nitrate - API are a good brand and widely available.
You said that your water perameters were 'fine'. Could you please post the exact readings?
What size is the crowntail at the moment, and how long have you had him in the tank for? At the end of the day, you're not going to be able to keep a crowntail in a tank that size and expect him to lead a happy and healthy life I'm afraid. These fish are huge waste producers and will pollute the water in a tank that size extremely quickly.
Anyway, if you could post your exact water readings that would help :)