This weekend I made a water change using rain water I collect in a tank. Some time after I added the new water I noticed the water became cloudy as if snow was falling and on the bog wood I had a white dusting. Anyone seen this before, I remember seeing this once more before. This is what I've done: Took about 50% of the water out. Placed 2 Crypto fertiliser tablets (first time use) Added water (was some what cold about 20°C since I did not heat it long enough) I noticed Amano shrimps were darting about in an unusual manner Also lots of MTS came out and seemed to be grazing the white dusting on the bog wood Unfortunately the camera battery was discharged, if I see it next time I'll take photos.

Difficult to say exactly what this could be but you could be experiencing a bacteria bloom, which is not great news in an established tank. If possible check your water for ammonia presence. The large water change along with the temperature change might have triggered this.

I doubt there is ammonia, since nitrites are 0ppm and nitrates are between 100 and 250ppm so I guess conversion is taking place.

my two theories are either some fungus in the bog wood came out, or some fish or the shrimp dropped eggs prematurely and they disintegrated - but it was too fine to be eggs

Is the water still cloudy now or has it settled down? If it is still cloudy than it is likely that you have an imbalance in your water conditions and there has been a surge in the bacteria count to combat this. This could have been caused by some impurities in the rain water. If your aquarium is well established, than you should be able to bring it under control with some more partial water changes, though you should be very careful not to overfeed during this time.

Everything was back to normal within 2-4 hours. It was just a strange effect.

BTW conductivity of aquarium water was about 500µS/cm the rain water I added was about 10µS/cm