Even with clean water, 0 nit, 0 amo, good ph I still have to worry about the feeders I buy transferring icks to my tank.The easiest way to PREVENT ICK is too have clean water w/ little nitrates (5-20) no nitrites and NO AMMONIA!!
IMHO dirty water is the biggest stress to fish they can have, its like US having dirty air and suffocating!!
Once u test and figure out ur nitrate peak (bio load capacity) you can see how many times per week to change. That way ur nitrates will barely fluctuate and ur fish will grow faster and have better immune system properties!!!
#1 S. Vettel