It see 2 possibilities.
1. You had 4 clownfish for 6 months. then you added 4 more fish. That's a significant bioload increase. Possibly an increase in Ammonia?
2. you have 4 clownfish and 2 damsels, all part of the Pomacentridae family of fish and quite territorial. Chromis fish are from the same family of fish, but they are a peaceful schooling fish. They both may be pestered to death with 4 clowns and 2 damsels in a small 25 gal