I would not add too much ferts for a plant hold system.
Often since all the leaf loss and decayed plant biomass are a big issue, I would not add much, rather doing large water changes will help more.
So when you get a load of weeds in, do water changes often. Then once the plants have acclimated, then add more ferts.
Adding nutrients/CO2 in the sump is the best/only real solution.
I'd crank the CO2.
I'd use a simple hard plumbed valve for draining and filling. That will make things simple and ease labor wise. While tanks are draining/refilling, that allows you time to tend to the tanks, remove floating leaves, prune and srub off any algae.
You will not able to avoid that part.
Dosing can be done 2x a week and so can water changes.
That would be a rather simple routine. CMS and KNO3/KH2PO4 are cheap.
Water is cheap. Baking soda/MgSO4, CaCO3, CaCl2 etc is cheap also.
Assuming a total volume of 300 gallons, you can likely spend 2-4 hours total time tending and water changing if you do 2x a week routines.
You'd dose after each 30-50% water change 2x a week:
KNO3: 2 teaspoons
KH2PO4: 1/3-1/2 teaspoon
Traces: (2 tablespoons in 500mls of H2O) 50mls
GH as needed to add 3+ degrees
KH as needed to add to 3 degrees.
CO2 as much as you can and not kill the fish.
This will keep the tanks in very good shape as well as plant health.
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