Hi all,
First up I'm very sorry for your loss, but you need to know some other details before you can make any sense of a pH reading.
Parameters
The parameter you really need to know is the conductivity of the water, and the other parameter that would be most useful is the carbonate (dKH) buffering, particularly as you say that your test strip read "
very low dKH".
pH meter
An electronic pH meter is a useful acquisition and the one in the link looks a good one for the money (I certainly wouldn't buy a cheaper model than this), but these aren't meters that you can just dip in and get a reading, they need calibrating with pH4 and PH7 buffers before every use, and the reference electrodes need to be stored in the correct manner to avoid damage to the membrane.
pH measurement
Because pH is measured on a log scale, and uses the ratio of H+ to OH- ions as a proxy for the acidity/alkalinity of a solution, not all pH readings mean the same thing. This is why I would like to know the conductivity of the water, if this value is below about 100 microS, pH is pretty meaningless.
Adding the oyster shell was discussed in another thread on here, I was going to do this anyway. I am open to suggestions for other causes.
Oyster Shell Grit
I probably suggested the "Oyster Shell Grit", it is the aragonite form of CaCO3, which makes it more soluble than the calcite form, and it is a cheap and easy way of adding some dKH/GH to your water. We may want to do this if we think that the lack of carbonate ions may cause problems with the CO2/carbonate equilibrium. However we know that carbonic acid (
http://en.wikipedia.org/wiki/Carbonic_acid) is a weak acid, and that the changes it causes in pH are not the same as changes in pH in a buffered system.
What killed your fish?
Having said all that I would be absolutely amazed if this was a pH effect. If you add CO2 I would be fairly happy they've suffocated, and if you don't I would expect that it is related to lack of O2 reaching the filter media, due to the higher bioload. You said they died over-night, and that would strongly suggest it is a O2/CO2 ratio problem.
Level of planting
Some measure of how heavily planted the tank is would be useful. If the tank is very heavily planted the likely cause of the fish suffocating is that the extra bio-load meant that the exchange of gases through the gas exchange surface was inadequate, and CO2 built up to levels which proved terminal whilst the plants were respiring, but not photosynthesising, due to the O2/CO2 ratio and Bohr effect.
If the tank is lightly planted, the filter media became oxygen depleted due to the extra ammonia produced by larger bio-load, because the conversion of NH3 to N02 to NO3 is an oxygen intensive process. It also utilises carbonates, in this case the low dKH measurement may be an indicator of this problem. The reason this doesn't happen in more heavily planted tanks is that the plants preferentially take up NH3 and it doesn't end up in the filter.
Test Kits
All other readings are fine including ammonia, nitrite, nitrate and GH.
You honestly don't gain anything useful from these tests, to accurately measure these parameters you need lab. grade kit that needs frequent re-calibration and then you need to analyse and prepare your samples using appropriate protocols.
I've got access to lab. kit with ion selective electrodes, an atomic absorption spectrophotometer and both GC/HPLC and I still wouldn't know what had killed the fish in the same situation by using analytical means.
Hope that makes sense, cheers Darrel