My BF just lost a clownfish and he's really sad, wondering why... I don't know what to say because his parameters look alright to me, though I only know freshwater fish...

The amonia is 0, nirtites 0, nitrates 20, salinity 1.021, ph 8.2. Tank has been cycled for about a month; fish were added after cycle was complete. Tank is a 12 gallon nano. Only other fish in there was another ocellaris clown and some snails.

Any thoughts?