### Water Change Calculator by MOA

Update: Spreadsheet Changed To Account For Earlier Discrepancies

While cruising the internet I noticed an online water change calculator (TheAquaTools.com) that predicted how various chemical levels (nitrate, phosphate, etc.) in the aquarium would change in response to a new water change schedule. I very much liked the concept and admired the fact that it represented an improvement on some of my own formulas. No less, I also noticed a few fallacies in the aforementioned calculator and noted that its abilities were a little limited. Consequently, I just had to build my own to see if I could do better, and I think that I succeeded in this aim. My version of the water change calculator is below (see attachment).

### The Original

The original water change calculator was very well-designed, simple, and effective. I, no less, am never simple, never pause for design, and do not stop at effectiveness: I thus had to tinker with the concept. In using the online version, I noticed the following limitations:
1. It can only handle one scenario at a time.
2. It requires the users to know the aquarium's daily rate of increase.
3. It actually skips important values in that it graphs the data as single points per day (with lower points supplanting higher points). As this is the case, it actually presents data that indicates that the amount of waste falls before the day of the water change is reached.
4. It deals with a single cycle, not accounting for one cycle inside another (i.e., doing a 10% water change between every 40% water change).
5. It does not address how the user conducts their water tests and thus has a greater chance of over- or underestimating the result.
6. Lastly, it does not predict a long-term trend. Instead, it presents a value based on how many days the user arbitrates. This value may be a minimum value, is never a maximum value (due to its selection of minimum values over higher data points), or some other value that may occur anywhere along the curve.
These limitations are part of the nature of the beast in that simple models that are easy to create (and use) are often just that--simple. They do not account for many details and thus are going to have implicit constraints that really cannot be overcome unless the model itself is extended. This is the approach that I chose, I expanded the fundamental model. Nonetheless, I have to admit with a certain embarrassment that the creator of that online calculator did point out a major flaw in some of my own computations: I have not addressed the incoming water as a source of waste until I saw it being done in that online calculator. Heretofore, I have only been concerned with what the aquarium produces itself, but some contaminants, like nitrate and ammonia, are sometimes found in the water source in addition to being an effect of biological processes.

I had to rethink my fundamental recursive equations.

### My Presentation

My version, concisely, addresses the issues I noticed in the original. More specifically, my spreadsheet version has the following features:
1. Waterchange1 addresses the unique cases of ammonia, nitrite, nitrate, phosphate, and one extra (user-defined) case.
2. My spreadsheet asks users to enter data about their past water change data, cross-references that data with the users' chemical conditions, and defines the daily rate of increase within the spreadsheet itself. It does not require the users to know the specific daily rate of increase.
3. Waterchange1 graphs both the high and low values for each day and thus does not contradict itself.
4. My version of the calculator allows for the possibility of one cycle within another. Unfortunately, further nesting is not possible at this time.
5. It does address how/when the water is tested and adjusts the initial condition to match this data.
6. Lastly, waterchange1 does evaluate the maximum limit of the major cycle, allowing the aquarist to examine the long-term effects of his/her choice of water change schedule.
How well I did in creating a "better mousetrap" is something that I will leave in the users' hands. I look forward to the various ideas people will send my way as a result of this new spreadsheet concept.

Water Change Videos
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MOA Fishkeeper,
Mar 13, 2010, 8:11 PM