My mother has a Koi tank. It is 29 gallon and is now housing one 6-7 inch koi, and two 6 inch common plecos. Resently, she had 10 koi die and she doesn't know why. (This is me banging my head) She says they were all doing fine and then they were dead. It's about two weeks later and I finally did water tests and these are the results.

Ammonia 0ppm
NO2 0ppm
ph 7.4 (8 out of the tap)
GH 520 (340 in another tank)
KH 30 (60 in another tank)

The filter is an Aqua "something" 200. Canister filter HOB, filter is 2 months old.

These tanks use the same tap water! Why the difference and what if anything should I do? (Besides building her a 100 gallon indoor pond to house two koi!)

Anita

Sorry,I dont really know(anything)
Isn't the PH supposed to be lower than that to crash??

8.0 to 7.4 is a very big drop, and combined with a low KH, it does indeed indicate there was a severe pH problem.

Remember the pH scale is logarithmic, meaning it jumps in multiples of ten. So a pH of 7.8 is 10 times more acidic than one of 8.0, a pH of 7.6 is 100 times more acidic and a pH of 7.4 is 1,000 times more acidic. And if it dropped any lower - into the 6.0's say - then you can imagine how badly the fish would be affected.

Why did it only happen in one tank? Koi are HUGE waste-producers: even more so than goldfish. The organic acids produced in their urine and by the decay process build up very quickly and can swiftly drop the pH hour by hour until it reaches fatal regions. It is for this reason that koi are never ever kept in tanks (except very temporarily when they are babies); they are true pond fish, needing around 1,000 gallons each to grow and thrive. Plecos are also large waste producers. The combined wastes of so many tank-busting fish in so small a tank could easily have overwhelmed the buffering capacity of the water and caused a crash.

You will need to boost the buffering capacity - the KH - of the water to well over 100 to stop the pH dropping again. You can either do this by adding baking soda to the water (a teaspoonful at a time, testing the pH and KH after each addition and making sure you also buffer the new water during every water change) or by adding a lot of crushed oystershell to the filter. Either way will work, but the KH and pH will always need close monitoring in future; I would say at least 3 times a week.

Really though, the problem here is gross overstocking. Unfortunately your mom will continue to lose fish until this is sorted out.

Hope this helps.

Thank-you, emmahj! This is what I was thinking as well but I didn't have the words to explain it. Plus, like Whirlwind said, the ph is still in the ok zone for goldies, it was the drop I was unsure about.

I don't think I can test 3 times/week and the man "taking care of" these fish would never care enough about fish to add baking soda to the water at changing time. (He says he changes the water but I don't think he does.) Do the oyster shells need to be in the filter, or is just in the tank ok? (And would oyster shell be enough without the baking soda? I thought the oyster shell only started to disolve at 7.4 ph.) My mother has a AquaClear HOB filter sitting unused, could I put the oyster shell in there or should I take it for one of my tanks?

I have so many questions but I love to learn new things. I eat knowledge like my goldies eat peas!

Anita

In the filter really, so that water passes over the grit and is adjusted; if just placed on the bottom of the tank, it still works but not as effectively. Don't forget that the grit doesn't last forever though - it will need checking and replenishing from time to time (and it varies how often you need to do this depending on how much grit you use, the size of tank, the natural pH and KH of the water, etc.).

What you could do though is put some seashells in the tank itself; they are pretty and decorative but they are also pure calcium carbonate and will help a little to keep the pH and KH up.