Water changes, dosing elements, and water volume

michaelr7170 · Jul 26, 2017

I would like to get everybody's thoughts on something that's been on my mind ever since I upgraded my reef to a much larger system. I had a 90 gallon, with a 30 gallon sump for about two years, successfully maintaining a mixed reef by doing ~10 percent water changes weekly, with 2 part dosing to maintain my calcium and alk levels. 10 percent isn't a whole lot of saltwater to be mixing and using each week, so I never really thought much about it.

In January of this year, I upgraded to a 180 gallon tank, with a 40 gallon breeder used for my sump and a 75 gallon refugium also plumbed into the system. So, I now have about 250+ gallons of water in my system, suddenly water changes aren't so small!

But this got me thinking, shouldn't water changes be a percentage of your display tank volume, and not your entire system? I mean, if I plumbed my display tank into the ocean, well you see what I'm getting at here...

This also relates to dosing elements for example, most instructions say to dose X mL per L of water volume, again wouldn't it make sense for this to be percentage of display tank volume?

Texas Reefer · Jul 26, 2017

I think it's percentage of all water in the system. The elements are distributed throughout the system, they do not just stay in the display. But remember rock, fish, sand, etc displace water, so there is not 180 gallons of water in your 180 gallon tank.

Randy Holmes-Farley · Jul 26, 2017

There are two different issues of this sort of "calculation".

The first is if something such as alkalinity is low. If you know you want to boost it by X dKH, then the amount you need to dose must be based on the actual water volume since the additive enters the entire volume.

The other use of the term is much less accurate. For dosing something that is trying to match demand without really knowing what the demand is, you can only crudely estimate the need. Like a trace element that is recommending 1 mL per 10 gallons. I doubt any estimate can get close enough to usefully distinguish between total system volume and display tank volume, but you are right in that the demand is largely from the display, and having a huge sump filled with water doesn't increase the demand. Of course, some types of demand do come from things outside the display tank, such as a refugium that is growing macroalgae.

Ty Hamatake · Jul 26, 2017

@Texas Reefer is right, water changes and ESPECIALLY dosing are based on total water volume. When I filled my tank I did it painfully slow keeping track of how man five gallon buckets I used to get as close to exact water volume as possible. If you were to dose only your display volume you would be under dosing by almost 30%. I'd say this is most important when dosing medications.

michaelr7170 · Jul 26, 2017

I understand there are definitely situations where you need to dose based on the entire water volume, as in a situation where you want to hit a target concentration of a certain macro/micro element, but as @Randy Holmes-Farley mentioned, demand doesn't change based on addition of water volume.

For example (using basic/easy math), lets say you have a 100 gallon system that uses 1 unit of calcium per day, and a 10% / 10 gallon water change every week was enough to maintain your calcium. Now lets take the same livestock, place them in a 1000 gallon system, they are still going to consume 1 unit of calcium per day. If you keep to the 10% water change per week rule, which in this case would be 100 gallons per week, you are assuming that suddenly your livestock is consuming 10x the amount of calcium! I understand it's not as cut and dry as this, as there are other factors at play as Randy mentioned, but just trying to make a point.

Ty Hamatake · Jul 26, 2017

Agreed, demand is everything. I think I was just confused about your bit about using the display volume to determine water changes/dosing.

Empress · Jul 26, 2017

Is there a calculation for displacement of water for sand, rocks, powerheads, overflow box, turf scrubbers, return pump, plumbing, skimmer, and whatever else there may be in the whole (wet) system?

Wow! I've never realized how much stuff is in my tank until just now when I typed this!!

Makes me wonder how much actual water is in my system. Does anyone know of a simple way to calculate all this?

UNCFAN · Jul 26, 2017

That does bring up a good question - how do you calculate actual water volume in a tank with a known value of rock and sand?

Empress · Jul 27, 2017

UNCFAN said:
That does bring up a good question - how do you calculate actual water volume in a tank with a known value of rock and sand?

That's what I'd like to know!!

michaelr7170 · Jul 27, 2017

Ty Hamatake said:
Agreed, demand is everything. I think I was just confused about your bit about using the display volume to determine water changes/dosing.

Yes reading it over, I could have been more clear