Ph Crash

Hi Chris

A pH of 6.0 isn't necessarily unhealthy - although it is on the lower end many fish are quite happy down to 6 or thereabouts and some even lower (e.g. Cardinal Tetras) and fish are generally quite adaptable as long as it doesn't go too far out of their ideal range + there isn't a sudden drop and/or the pH doesn't fluctuate regularly.

I'm sure I've read somewhere that tanks often drop in pH as they become more mature so perhaps wasn't a pH crash - just maybe your tank dropping to it's 'natural' pH level which had previously been kept higher owing to the regular water changes? Did you do a water change before testing the pH?

Ah, forgot the API tester doesn't go lower than 6 so the true reading could be lower as you say. People on here generally advise against any chemical intervention wherever possible and I think doing something which would shoot the pH back up would potentially do more harm than good - a gradual increase would be better.

Your tank is quite large isn't it so the 25l change you've done already is fairly inconsequential, in which case I'd probably do your 50l change straight away and then test again to see how much difference it makes.

The end product of the nitrogen cycle (nitrate) is acidic as as various hormones etc secreted by the fish. With medium or high KH there is enough buffer in the water to resist a drop in pH. But as you've found, with your low KH there isn't enough. This is what happens in 'old tank syndrome', where water changes are done infrequently, even where the KH is high.

You need to do small daily water changes to get the pH back up - too big a change in one step is harmful, and they've just been through a big drop. And next time you go on holiday maybe do a few larger water changes before you go to add a bit more KH than usual.

If you want to be sure of avoiding a drop again, you could give some though to adding remineralistaion salts. These are used with RO water but can also be used to add a tiny extra to non-RO water such as yours. You wouldn't need much, just enough to boost your KH by 1 or 2 degrees. The downside is that they would also increase your GH and pH slightly.

I completely forgot about crushed coral 
Yes, you can use that. Or a chunk of limestone rock as decor.

I found a bag of crushed coral a while ago (at a shop that has since closed) It's irregular white lumps like this. The biggest pieces are around half an inch long. Your shops may have something different. Just put it in a mesh bag. Other people use coral sand as the substrate, but that's mainly for those with Rift Lake cichlids.

As for how much, I'm afraid that's a trial and error job. Start with a couple of tablespoons and work up from there to get the desired result. It will increase your GH and pH as well.

As a very approximate idea, I filled 2 tubs with water, about 3 inches deep. Into one I put a layer of crushed coral maybe half an inch thick, possibly less. After a week, I compared the 2 tubs (plain water first and water with coral second):
pH - 7.4 and 7.6
GH - 8 and 10
KH - 3 and 5.

This gives you an idea of how big an effect it can have.

The only thing to be careful of using coral like this is that when you change the chemistry of the water, it is different from your tapwater; you need to avoid big water changes as that will alter the chemistry of the tank water. Smaller, more frequent water changes are safer for the fish.

just a little update
i did a 50 liter water change last night which raised it up to 6.4 , so when i get home later i will test again and do another 50 liters and test after to compare results and have just ordered some crushed coral from ebay so can slowly add that when it arrives

*phew*