Hello I have recently gotten into the saltwater hobby and had taken on two clownfish they were doing great for a couple of weeks I checked ammonia, nitrite, nitrate, ph, and salinity once a week I do 10% water changes once a week as well they seemed to be normal until I came home today both of my clowns were on the sand dead I have a hermit crab and a small frag of gsp they both seem to be perfectly fine I tested my parameters and everything was at zero I recently like 2 days ago did a water change and had added a small amount of seachem prime like I do to my 55 gallon cichlid tank I think they had plenty of oxygen because the filter I have is rated for double the size of the tank any ideas on what could have happened?