nitrate level

Question
what is a good level to keep my nitrate level for clown loach it at 40 now

Answer
The nitrate level, unlike pH or hardness, is always preferred quite low. Under 30 is acceptable, but basically the lower it is the better. You should be more worried about the nitrite and ammonia level, as these are very poisonous and must be kept at 0.0. Putting live plants in the tank lowers the nitrate quickly.
Best wishes,
Kathryn