Oh, that's quite a large amount of fish in a relatively small tank, I'm afraid. That might even be the answer to the problem. Clown loaches in particular can grow to about a foot long (smaller tank will stunt them and shorten their life considerably). Plus they need to be in groups of 6 or more in order that they feel comfortable.
Apart from stocking, can you tell us any info about Ammonia, Nitrite, Nitrate, pH, temperature etc? In a tank this heavily stocked you may find a large amount of waste leading to poor water conditions. Did you fully cycle your tank before adding fish?