i am not sure how much flow That eheim has but with my canister for ease of install, i put my diffuser under the intake so it sucks up the Co2. I place it as far below as possible without losing the any co2 to the surface(the more time it has to dissolve to smaller bubbles before it gets sucked up the better). You could also buy an inline diffuser for the canister. The heater I would place somewhere near the in or out of one of the filters so you dont have any cold spots. Drop checker I would say place it out of the main flow of any filter. Filtration overall seems like a lot of current. I would play with it to see if is too much. Set the canister up first and see. Also if either of the filters are being used now keep the filter material in one of the filters for bacteria( not sure if this is a new setup) Well thats prolly more than enough info to hopefully help somehow?