Sudden Fish Death

Question
I have a 20 gallon FOWLR tank.  I have had a clownfish, Fox Face Lo, Chocolate Chip Starfish, a Copperband Butterflyfish, two Turbo Snails, a Banded Coral Shrimp, and four blue-legged hermit crabs.  All have suddenly died with no warning excpet for the Clown and Hermit crabs. Nothing changed and they were all eating well and not showing any signs of distress when they died.  They all died within hours of each other.  I decided to wait on getting more fish and just kept the Clown and hermit crabs.  They have all been doing fine.  I added the Starfish and he was doing great. I checked on him before dinner and he was alive and eating well.  After dinner I came to check on them and the Star was dead.  After both times I checked the water and everything was in the ideal range.  I do a 20 percent water change once a week and the tank appears healthy and the lady I get my fish from has no idea whats going on.

Answer
Hi Lynzie,

Sorry to hear of your losses.

My thoughts would be: perhaps your bioload exceeded what your aquarium filtration could handle.

Tip: for an aquarium of only 20 gallons, keep only a few fish, that are more appropriate for this size aquarium.  For example: the minimum tank size for a Copperband Butterfly fish is generally 50 gallons. The minimum tank size for a Fox Face is generally 70 gallons.

I wish you the best with all your future saltwater fish endeavors.

Sincerely,
David