Are Large Water Changes Killing Fish? Small vs Large Water Changes -
- Thread starter tarheel96
- Start date
This happens because the water's alkalinity or KH has been eroded by the nitrification process. The nitrifying bacteria need and use the carbonate alkalinity (as HCO3) to turn ammonia into nitrate. This is one of the main reasons water changes are performed ... to replenish/restore KH and prevent KH from being eroded/pH from crashing.
Nitrosomonas
NH4+ + HCO3- + O2 + Phosphorous + trace elements -> bacterial biomass + NO2- + H+(acid)
Nitrobacter
NO2- + HCO3- + O2 + Phosphorous + trace elements -> bacterial biomass + NO3
When tank pH is at 6 the water's KH will have been eroded almost completely. At KH 0º, pH will usually fall into the low-mid 5s. This is what happens with Old Tank Syndrome but it doesn't necessarily take a long time to happen between water changes if the water is very soft with low alkalinity (low KH).
Check you tank for total ammonia before performing a large water change.
My water falls from 9 -> 7.2 in 24 hrs. It stays at 7.0-7.2 in all my tanks with Kh of 1-2 in all of them... Even just leaving it in a bucket for 24hrs gives me that
You don't see a drop in pH due to KH erosion between water changes? My water is about the same KH and I have to add bicarbonate between water changes to keep KH/pH from crashing. Some of it might depend on bioload though.
I don't have tests for it, a friend came over and got 1-2 Kh on all my tanks. The drop is straight from tap...
checked my tank ammonia its at 0ppm. so should I do a 50% water change or a 30% water change everytime?
If you're used to doing smaller water changes and/or it's been a while since the last wc, I'd start with smaller WCs first and then gradually increase the size. So you might start with a 30% wc now, then a 50% next week, a 60% the following week, and a 75% in the weeks following that (as needed).
50% now probably wouldn't cause a problem but the other way would be safer.
Waterchanges (if your tank is cycled) is not about removing ammonia.
Water changes are about reducing nitrate, and nitrate precursors, and keeping the alkalinity (the waters buffering capacity) of your tank stable.
Water changes are about reducing nitrate, and nitrate precursors, and keeping the alkalinity (the waters buffering capacity) of your tank stable.
Usually I do 70% once a week, straight from the hose. The fish seem to like the cold water swimming directly into the hose. As mentioned big temp swings cam be an issue, only once in the last 15 years the water wad maybe a little too cold. The fish settled on the bottom for an hour or so, they were fine after that. The good news was I finally could get an accurate measurement. 
