So I wanna do a X% water change - calculation and water meter?

[oliverk]

So I want to do an X percent water change.

Do I include display tank; or
Display tank plus filtration liquid capacity; or
(Display tank - decorations) plus filtration capacity; or
(display tank - decorations) plus (filtration capacity - filtration media) ~ liquid in the system; or
other?

My initial thought would be (Display tank - decorations) x whatever percent I want to change

Thinking that liquid in the filter system is not being polluted by the fish, rather it is being filtered- made better, treated......

Most conservative, of course would be drain the percentage of the entire system and replace, but to the extent the filter system is large capacity that could be way over kill.

Any one use a meter to measure new water flowing into a system by drip? How much extracted, or is everyone got a bucket with a line on it that says this is X gallons?

If using a meter, what is an effective, reliable, cheap with a good easy reset so you can accurately measure what is going in (thinking of measuring the auto top off amounts of new water into the system)? Thinking if I have over the last 24 hours put in 15 gallons, or over the last week put in 40-60 gallons into the system, how much more do I need to change to complete my target X percentage water change.

Want lots of good clean water for the fish, but want to minimize waste (erring a little on the side of clean for the fish)

[discuspaul]

Not sure why you would feel the need to want to be that precise with your water change measurements.
It really can be made a lot simpler than that. An approximation would do, i.e. a best guesstimate taking account of volume capacity of the tank, less a rough estimate of the quantity of water displaced by décor & other items. Waste would be quite minimal.
Have I misunderstood your post ?

[sdrexler078]

I think you're over complicating things. Just K.I.S.S

[afriend]

When doing water exchanges, there's two basic ways to accomplish it: the direct displacement method or the slow drip overflow method. The direct displacement is accomplished by first removing X gallons of water from a tank that contains a total of Y gallons of water and then adding X gallons of new water back into the tank. When finished the percentage of new water added is X/Y times 100%. The slow drip overflow method is accomplished when the tank has an overflow built into it. As the new water is added to the tank, it mixes with the tank water and some of the tank water and some of the new added water is flushed out of the tank by the overflow. Thus the percentage of new water exchanged by the slow drip overflow method will be less than by the direct displacement method. Here's a link that can be used to calculate the percentage of new water added to the tank when using the slow drip overflow method.

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0CCEQFjAA&url=http%3A%2F%2Fwww.angelfish.net%2FDripSystemcal c.php&ei=j3FCVM-qDYbt8QGtqYHgDg&usg=AFQjCNHIuYdU3igpEm3e9Es7gJb4tdXhGg&sig2=VP621TBY5i-VhlQ37-uAjA

Hope this is helpful.

Paul