Water change volume question.

[EpisodeMnH]

So I have a 32.5 gallon, now when guidelines say to do a certain percentage, I’m not sure if you’re supposed to use the actual tank size, or if you’re supposed to estimate actual water volume and deduct from there. But in any case I’ve always done my math based on an even 30, even tho actual water volume is probably moreso somewhere in the 25-30 gallon range.

I have heard that if you do weekly changes, you should do 10% water changes, and if you do every 2 weeks, you should do 20%. For a while, I was doing a consistent 5 gallon water change weekly, which is actually more like 16%, since 10% is 3 gallons. This more or less kept my nitrates stable, staying about 15-20 PPM, but that’s obviously pretty high, and there would sometimes even be spikes. It would never quite bring it down. To add to this, phosphates were consistently 0.50-0.60.

Fast forward to now, being about a month since I ran a 6 week course of Reef Flux, and I’ve changed my tank maintenance up because the Reef Flux treatment spiked Nitrate to 25 PPM and Phosphate to 0.70+ PPM due to having no filtration during that time. So I started running PhosGuard, and for 4 weeks now, I’ve done 10 gallon water changes weekly, which is a 33% water change in my case.

I’m now at a point where my phosphate got down to 0.14 PPM at it’s lowest, and nitrate was 2.0 PPM as of last night immediately after a water change. Both numbers are probably the lowest I’ve ever seen in my tank.

Last week, I tested nitrate both immediately before and immediately after the water change, it went from 12 PPM to 7 PPM, so it dropped 5 PPM. This week, I forgot to do the before test, but immediately after, it was 2 PPM. So if we assume that the 10 gallon water change removed 5 PPM again, then you could estimate that the nitrate more or less stayed at 7 PPM all week, and this stripped it even further. I know 2 PPM can be almost too low since I run a full mixed reef with all 3 coral types and softies and LPS benefit from a bit more nutrients, so I was aiming more for that 5-10 PPM range. So obviously for now I’m done doing such big water changes because I don’t want to zero things out now.

My main question is how I should go about water changes going forward to try and keep parameters in this range that they’re currently at. 10 gallons weekly is clearly too much so should I go back to 5 gallons a week? As I said, I was doing that for the longest, but it was never quite dropping my nitrate and phosphate and I was staying consistently high on both. But now maybe that I have things sorted out better and cleaned up, the consistent 5 gallons would keep them in this range? Since I’m no longer trying to go any lower.

 

[emmysnewtank]

Just keep it simple man. That’s too much math. Change a couple gallons. Coral look happy? All good.

 

[Mr. Mojo Rising]

That’s way too much thought into it.

I have a 32 gallon I do either 5 or 10 gallons weekly change because I have 5 gallon buckets and I like doing water changes. On my 15 gallon I do 5 gallon changes weekly. Whatever works for you and the tank.

 

[Miami Reef]

If 10% is lowering your nutrients too much, then try going back to 5% weekly.

I agree that it’s not rocket science, but rather more of an art.

 

[Tim McLellan]

Way too many numbers. Keep it simple. My first year I changed roughly 10% per week with testing. Eventually things seemed to not change much each week and it started to feel like overkill, so I backed off to 10% with testing every 2 weeks. So far so good. Of course I battled the uglies during the first year, but things seem pretty stable now.

 

[Randy Holmes-Farley Verified Community Expert]

I would advise that trying to keep alk, calcium, nitrate or phosphate perfect by regular water change is generally very challenging and are not the main reason to do water changes (IMO). This is what I think is most useful:

Water Changes in Reef Aquaria by Randy Holmes-Farley - Reefkeeping.com

Conclusion
Water changes are a good way to help control certain processes that serve to drive reef aquarium water away from its starting purity. Some things build up in certain situations (organics, certain metals, sodium, chloride, nitrate, phosphate, sulfate, etc.), and some things become depleted (calcium, magnesium, alkalinity, strontium, silica, etc.). Water changes can serve to help correct these imbalances, and in some cases may be the best way to deal with them. Water changes of 15-30% per month (whether carried out once a month, daily or continuously) have been shown in the graphs above to be useful in moderating the drift of these different seawater components from starting levels. For most reef aquaria, I recommend such changes as good aquarium husbandry. In general, the more the better, if carried out appropriately, and if the new salt water is of appropriate quality.

Calcium and alkalinity, being rapidly depleted in most reef aquaria, are not well controlled, or even significantly impacted by such small water changes. In order to maintain them with no other supplements, changes on the order of 30-50% PER DAY would be required. Nevertheless, that option may still be a good choice for very small aquaria, especially if the changes are slow and automatic.

 

[exnisstech]

10%, weekly is kind of a norm that gets thrown around as gospel. Each tank is different so you'll have to find a balance for your tank but it sounds like less for you may be needed. I have two tanks and have switched to daily water changes. One is about 40 gallons and I do 1g a day. The other is about 200 gallons and I do 4g a day. I dose kalk and 20 ml 2 part in the large tank to keep alk stable at 8.0 On the smaller tank it has always run low nutrients and since adding a lot of SPS frags I have begun dosing N and P to try and keep them balanced and detectable. I also have to dose 2 part to keep alk up which is kind of challenging as the frags grow. Sorry for being long winded.