All right, mathematicians...reason me this one....


Staff member
RC Mod
...because math skills are not mine.

100 gallon tank, 10 gallon water change per day, as follows:
I dip 10 old gallons out of the sump.
I put in 10 gallons of new salt water.
I do this for 10 days running.
At the end of ten days math-deficient me believes that I will have done a fairly gentle 90% water change, on a tank that has been through bad times.
Is my math squirrel?


Super Best Friends!
Premium Member
If new water didn't mix with old water, that math checks out. It mixes, so welcome to the math suck.

I'm sure there's a formula, and I'm pretty sure I worked through it in general chemistry in college right before I forgot how to do it on the test.

That said, we never want anything to happen fast in our tanks. Cutting 10% by 90% over time will eventually get you where you want, but it won't happen this week. Coral and fish will probably thank you, though.

Math nerds deploy! tell us how it works.


New member
Doing 10% water changes, you'll have about 35% of your original water in there after 10 days.

Think about it like reducing nitrates. You're going to reduce the level by 10% each day. If you start with 10ppm, after day 1, you've got 9ppm. Day 2 will be 8.1ppm, Day 3 will be 7.29ppm, etc...


New member
10% WC leaves you with 90% "œold" water each time, so multiply original volume by 0.9 for each water change to find difference from the starting point.

WC 1 - 100G x 0.9 = 90G
WC 2 - 90G x 0.9 = 81G
WC 3 - 81G x 0.9 = 73G

Sent from my iPhone using Tapatalk Pro


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Thank you all. This tank has been through, well, real bad times in 2020 qt'd from supplies except salt, etc, and me down with medical issues. So rather than start over, I ordered a massive reef cleanup crew (which is doing great work, even though minuscule snails) and started this program of daily water mixing and replacement, 10 gallons a swat, so my fishes are fine, the inverts are fine, and the tank gets a boost with nearly all new water. I never even have to stop the pump. It's looking way better even after 4 changes, (and the cleanup crew). I got a neat 'flat' water mixing container, meant to fit as a wastecan under a counter, and has a handle on one side. Would hold 23 gallons, and is nice in the limited workspace I have for doing this, and ten gallons of rodi and 5 cups of salt are not hard to measure out. I also acquired a 1 gallon marked-measuring pitcher, which is really nice. Items from Amazon. I begin to have hope for this poor tank.

Thank you all!


New member
Please keep going!

Please keep going!

Just want to say years ago you helped me soooo much by reading your threads, and I still look you up! What clean up crew, just curious, because I need to order, too. Thank you so much, and please keep going with your tank, I've had numerous mistakes, etc, with mine, but I enjoy it so much.
PS Did you skate, or is it for sea skates?

---figure skated, ice. Far from professionally, for sure. But loved it.---sk8r.
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New member
I hope you are feeling better. I used to read your articles as well. I just got back in to reefing again myself. My new build is a 150 gallon though it will be lps/sps. It's my biggest reef yet. I have been going over your articles again since I need the refresher. It is good to hear your tank is doing better.

Reef Bass

colors and textures
Yo! Still breathing, still reefing, still shooting occasionally. Been meaning to post some shots. You've been warned. ;)


Staff member
RC Mod
I had a bout with cancer, but thanks to chemo and good docs, am doing extremely well, so thank you. Weak as a kitten, and unable to do all I'd like to, but my water changing is having good effect. Even a bleached mushroom has turned up alive, and I'm feeding it amino acids (Brightwell) and floating food it can access through the water. Bristleworms, unseen for two years, are out and about. And vermetids, but even they are a sign of healthy life, and will diminish as time goes on. I sprang for Reefcleaners package for a 100 gallon, which provided me snails and hermits, tiny hermits, but agile and hungry, and laughably tiny cerith snails, with a few nerites---but even though I had to pick some of these microsnails out of the medium with tweezers (literally that tiny) they are vigorous and all over the tank, noshing nonstop. Funniest inclusion was my request for a fighting conch. I expected an inch long. They sent me Brunhilda the Monstrous, nearly 4" long, and mine is an overbuilt rockwork up on rock pillars, with only about 4" of clear sandbed between the rockwork and the glass (bowfront tank). I hesitated to put her in there, but those support rocks are wide-spaced and solid, and I finally reasoned that if she could dive under the sand (they do) she could maneuver and manage. So in she went, and she chugs along on the surface a bit, and is now underneath, noshing on unreachable debris. THere MAY be an earlier (smaller) conch still in there, and they may meet. As will a few big true Nassarius snails I ordered.
The stir of life and reappearance even of pest species is encouraging. My surviving fish are happier---feeding kept them fat and healthy but during the lean years of Covid, they had to make do on flake I could get online. Now its frozens, and they're deliriously friendly. So things are generally looking up. Thank you very, very much for the kind thoughts, and I'm on line regularly and hanging out, so let's all wish each other better supply and healthy tanks again!


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RC Mod
Another resurrection---just found this morning---a tiny patch of zoas that had been shoved off by a snail, now showing orange color and responding to the changed conditions. I'm giving the tank a shot of Brightwell's amino acid for corals daily and an addition of Coral Frenzy (old jar, but it awakes interest in everybody) and frozen food, including micro fish ova. Amazing what's just held on for basically two years, just waiting. I'm preparing my 6th 10 gallon water change for this 105 gallon tank, changes just about as fast as I can mix the water, and am now curious as to what else may awaken. My pile of holey rock makes a 2'x2'x3' maze in this triangular 3'x3'x3' bowfront, and when things disappear into it they've got a lot of maze to be in. I'd really like to see those zoas develop: they're a pretty lot, whatever their name is.
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