Is it ok to extend the period between water changes when running the EI method? Normally I stick to the 50% once a week but am hoping extending that to once a fortnight won't be a problem. I run normal doses plus 25% increase.
Plant growth is good but it's more the fish and shrimp I'm worried about al though dnot think i hAve ant thing to sensitive. I have cherry plus dwarf cory, pencilfish and glowlight danios.
Thanks in advance.
If you are trying to create more stable water parameters for the shrimp, waiting 2 weeks between 50% water changes will have the opposite effect.
The most any nutrient can build up doing 50% water changes is 2x the total amount dosed in between them. (this assumes zero plant uptake)
For example EI nitrates is 7.5 ppm 3x week, with a 50% weekly water change. That is 22.5 ppm dosed between water changes. Therefore, the most you will ever have is 45 ppm (again this assumes zero plant uptake for simplicity's sake) So theoretically you are going from 45 ppm down to 22.5 after a water change.
But stretch that out for two weeks, and you are going from "potentially" 90 ppm down to 45 after a water change. Big difference.
Like others have said, if you are going to do bi-weekly water changes, best to cut EI in half or thereabouts.
If the main concern is stable parameters, even better would be 33% weekly water changes with reduced EI. The most you can build up changing 33% is 3x the total dosage between water changes. So there is potential for a greater build up of nutrients, which you can adjust for, but a smaller swing in parameters after each water change.