if most of the bb lives in the filter and on the surfaces would large daily waterchanges with prime cause any ill effects?
Water Changes can cause you loss of BB so better to take it easy until your tank cycling is in check and the water clears. I love your tank layout, but big fish create a lot of waste and even though you think they're eating everything, they're probably blowing out a lot of food from their gills when eating pellets or something dry. That too can cause the planaria bloom if those particles are being trapped under rocks or under your tiles. For my big tanks I keep them bare. It does not look pretty at all but at least I know what's in the tank and can get to it easily if it needs to be removed. I think I've seen that combination of events so many times that I forced myself to go bare and increase my biological filtration, as well as mechanical filtration and siphoning excess food and crap out of the tank whenever necessary. With your bio load, I would definitely run a sump and the two cannisters, that way you can alternate cleaning schedules of the filters. This will also help you to do less drastic water changes and not risk upsetting the balance of your BB. I would not rely on cannisters alone to keep your ecosystem going.
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