HELP 4 FISH DEAD!

[veunknown]

i found today all 4 of my rainbow sharks all dead?? all of my other fishs in the tank all look good. this have never happen to me before, the tank has been up and running for years.

ammonia 0.25ppm i think this is cause of the dead fish
nitrite 0ppm
nitrate 40ppm
Ph 6.0

 

[Lupin]

Do plenty of water changes until your ammonia and nitrates are reduced. What size is the tank and what fish are in it?

 

[veunknown]

its a 50 gal i change out the water i have 3 tiger barbs, 2 clown loach, 3 bala and a peacock eel.