Tank size or water volume?

[JosephMCorbett]

When you are figuring out how many GPH for filtration, watts per gallon for heating, tspn per gallon for meds., percent of water to change, etc., etc. do you use the tank size or the actual water volume. I figure my 55 gallon tank has around 40 real gallons of water in it after you subract for gravel and decor. So when I'm figuring I usually use 40. Anybody else?

 

[YoFishboy]

I think that the only time it is important is when you are using strong meds - otherwise, I believe approximate is ok!

 

[RTR]

I use actual volumes for dosage, for partials I do something more than actual volume but less than nominal (one of my 55s is 41 gallons actual, the other 42 - a 27 1/2 gallon water partial would have the plants in hopeless tangle, ~22 gallons and it is not a hassle).

My filtration is always multiple units, and that ends up being based more on the current I want than anything else.