This note assumes that you have read the note on what OTS is itself as well as the article on general Water Change Math, if you have not yet read those, you may want to scan them for background which may not be reviewed here.

Old Tank Syndrome, OTS

Water Change Math

When the water we want and plan to use does not match at all well with the existing tank conditions, if we do substantial changes we will subject the fish to sudden differences in the osmotic pressure of the water around them. This stress may be so severe that the fish suffer shock and may even die. They are adapted, hopefully and usually slowly, to the water that they are living in now. Too much change too fast is a system overload. This situation on the boards is commonly called “pH shock”, but it is not that, even though the pH may well be quite different. It is osmotic shock from the sudden change in TDS (total dissolved solids). TDS includes GH, KH, nitrate ion (NO3–), sodium chloride (Na+, Cl-), carbonates and bicarbonate, and all the other dissolved materials that we do not measure (potassium, sodium, sulfates, phosphates, organics, etc.).

We avoid a stress or shock situation by staring out slowly, and with small changes each session. The fish can adapt to the small changes in osmotic pressure without undue stress. In the best of all possible worlds, we would do this with two or three days up to a week between partial water changes. But in the real world we are often too impatient for that. We can get away with doing partials at 12-24 hour intervals if they are small enough in percentage changed with each partial. The notes presented here are based on 24-hour intervals between changes. If some of these intervals are longer, this is not a problem other than extending the total time to get the tank back to a situation where partials may be done routinely without osmotic stress dangers.

Accelerated Change Schedule:
Change # Start % Old % Change End % Old % Difference
Baseline 100.00 0 100.00 0.00
1 100.00 10 90.00 10.00
2 90.00 10 81.00 9.00
3 81.00 15 68.85 12.15
4 68.85 20 55.08 13.77
5 55.08 25 41.31 13.77
6 41.31 35 26.85 14.46
7 26.85 50 13.46 13.30
8 13.30 50 6.65 6.81


This table varies from those shown in the general note on water changes only by the addition of the last column. The “difference” is in the percent change in the titer or concentration of the dissolved materials accomplished with that particular step. Although we show the increasing volume percentage changed each time, that increasing volume does not itself show the change in the original material for each step. That is shown by the last column, “Difference”. What we are showing here with the added column on the grid is that the change in original material does not jump a lot with each individual session. The first two sessions are small changes, both by water changed and material removed. The third through the seventh partial sessions are the effective ones, but the difference in material removed at each session is neither great nor large. With this sort of schedule, we can get the water back to more or less normal and desirable conditions with daily partials over just more than a week’s time. The fish will have little or no stress from each individual change. The eighth change is really only to get the tank water to the <10% original material situation I set as a guideline or benchmark in the general water change note in this series. In reality, the eighth partial could be set at whatever percentage change is to be used in the future as the ongoing standard or routine change. This is because the residual material after the seventh change is no greater and likely less than would be expected routinely.

As a concept, this is a bit harder to grasp than routine changes on an established but stable tank. Here we are trying to correct an undesirable situation and at the same time protect the fish and allow them to adjust slowly without excessive stress. We are in effect combining all the tables presented in the general water change math note into one. We are shifting from smaller to larger partial water changes throughout this particular change series. But we are keeping the osmotic change per session relatively small. This minimizes the stress on the existing fish by minimizing the pH, GH, KH, TDS change in any one partial down to levels which the fish are more likely to withstand. Obviously, if the fish show distress, do not stick to the daily change schedule. Instead drop back to every other day or every third day, or even weekly if it must be that. But weekly changes will not be doing as good a job on getting the huge load of pollution under control. The whole object of this exercise is to get the fish back into high quality water without killing them, or overly stressing them. Then we can establish a better water partial change schedule to keep them there, in line with the material discussed in the earlier articles.

Robert T. Ricketts, a.k.a. RTR


Please enter your comment!
Please enter your name here