question

[daniella3d]

It's spring, and there could be something in your tap water that might have done this. This is why it is best to use RO water that is filtered before putting it in the tank. What type of conditionner do you use? It might not be enough to neutralize seasonal desinfectant that the water company might use.

At least that is my guess, but a small 20 gallons with 3 fish in there and no skimmer, just a small powerhead, could be low oxygen as well.

Could be a disease. How long did you have that wrasse that died?