It would be better to increase the size of your once weekly water changes a little bit (maybe do 30% once weekly water change instead of 20% water change) and clean the substrate a little more often (like once a fortnight), but other than that my main advice is to treat the clown loach with an anti external bacterial med like Pimafix and increase the airation in the tank during the treatment
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Are any of the fish displaying symptoms like flicking/rubbing themselves againt objects in the tank or tend to hand around the surface of the water or breathing rapidly at all?
The clown loaches will eventually outgrow the tank, although on average clown loachs tend to only grow slowly at 1inch per 4months, how large are the clown loaches currently and what are the measurements of the tank (length, width, height)
?