How do you measure WC percentage?

[*Ci*]

I see people changing water at different percentages - 30%, 50%, 70% and other numbers in between, but how do you calculate the correct amount of water to take out?
Did you meter the water that the tank actually holds, including the displacement of hardscape and plant mass, and then measure out the amount needed for your chosen percentage and mark it on the tank?

I have been doing 50% changes by measuring the water height in the tank, dividing by 2 and placing a piece of tape near the back glass to indicate where I need to syphon to, but is this even accurate? By doing it this way, I don’t think it is accounting for the volume displaced by substrate, rocks and wood, and I am actually removing more than 50%.

Maybe it is unimportant overall, but am curious what others do and how they think about this aspect.

 

[Kwyet]

I haven’t been accounting for non-water contents of the my tanks, so it’s a guesstimation. I do put a little mark on the side of the tanks so I’m at least consistent.

 

[rzn7z7]

I think it should be based on the actual volume of water held in the tank, not the volume stated by the manufacturer. You can use a calculator to help determine volume and should account for the thickness of the glass, substrate, hardscape (if it's significant), and the fact the surface of the water is some distance below the rim of the tank. I doubt I'd deduct for plant mass, however, don't overlook the amount of water sitting in filters

In my case I have 225 gallon aquarium (72" x 24" x 30") and when I deduct for everything above then add back the approx. 12 gallons in the filters I come up w/ a volume of 190gallons, and that's what I base WC % and dosing on

 

[*Ci*]

I think it should be based on the actual volume of water held in the tank, not the volume stated by the manufacturer. You can use a calculator to help determine volume and should account for the thickness of the glass, substrate, hardscape (if it's significant), and the fact the surface of the water is some distance below the rim of the tank. I doubt I'd deduct for plant mass, however, don't overlook the amount of water sitting in filters

In my case I have 225 gallon aquarium (72" x 24" x 30") and when I deduct for everything above then add back the approx. 12 gallons in the filters I come up w/ a volume of 190gallons, and that's what I base WC % and dosing on

So how do you then remove the amount of change water you need? If it is, say, 50%, how do you measure out the 95g you need to remove?

 

[rzn7z7]

I use 100% RO water and can only make 48 gallons RO a week, which is a littler over 25% of 190 gallons...so, one time I carefully measured taking 48 gallons out of the tank and stuck a piece of scotch tape on the side to mark the level....nothing fancy but it works

 

[Mr.Shenanagins]

I guesstimate, but I also dose EI style so it’s not as much of an issue or concern.

 

[*Ci*]

I guesstimate, but I also dose EI style so it’s not as much of an issue or concern.

I also dose EI, front loading once a week after a water change, and was under the impression that 50% is the ideal to shoot for with EI. If I am consistently going over that mark because of the way I am guesstimating, won’t my accumulation start to gradually decrease? This is what I have understood from @GreggZ post:
Journal - Greggz 120G Rainbow Fish Tank - Part Deux!

 

[Pepere]

I measure down to a height that makes sense…. On my 29 gallons I have a piece of tape about 4 inches above the substrate. This gives the fish reasonable amount of space to not be overly stressed when at its lowest…

How many gallons remain in and among the substrate? I have no idea. But it is relatively easy to come close to calculating the water removed as there is very little mass above that line. Simply measure internal width, length and height to get cubic inches. 231 cubic inches per gallon…

I pull out 16.5 gallons in a waterchange…

 

[ElleDee]

I don't worry about the percentage so much as the amount of water removed because I calculate fertz and such based on that. When I was using buckets, I picked the amount I removed based on how much water I could comfortably carry in the two buckets. That was what was most convenient, and everything else is secondary to that. On my big tank I have tape that marks where I need to drain to.

 

[Burr740]

If I am consistently going over that mark because of the way I am guesstimating, won’t my accumulation start to gradually decrease?

Technically yes, but it absolutely doesnt matter as long as you stay consistent. All it would mean is that you have a few less ppm in the water than you think

All these numbers like wc percent, water column totals, weekly dosing ppm. These numbers are just reference points for us personally, from which to base changes on, to repeat for consistency, and for communicating our routines to other people

Plants arent going to magically do great because we accurately hit a certain number. It makes no difference if in our mind we have 20 ppm but its really 17

We are still gonna go up or down based on what the plants say. They establish what numbers are good or bad, not the other way around. Whatever number we have in our mind is just a reference point for ourself to go by

To the original question. Ive always use the distance from the top of the substrate to the top of the water level. Thats close enough without worrying about things like substrate volume or how much the hardscape takes up. See reasons above

When I was first starting out I'd put a little piese of tape or a tiny dot from a sharpie at the 50% mark, or 75% or whatever. Now days I want at least 50% and no more than 60. I just eyeball it which is plenty close enough. If I accidentally go over 60% it just means I need an extra dose of macros to make up for the extra water removed