Flashing can be caused by all manner of things including temperature fluctuations, o2 level changes due to algae respiration, hardness of the water as well as Nitrite, Ammonia and of course parrasites. The trick is knowing how to recognise the difference between signs of irritation or natural reaction to change and this will come with experience.
In your case, Nitrite is a problem probably due to a combination of over eager filter cleaning coupled with chemical treatments and this will cause some flashing. I have just installed a Nexus Eazy on my pond and Nitrite has gone up to 0.25ppm which is causing some flashing as I expected. It's a pain for sure becuase Nitrite irritates so the fish react accordingly.
Now that you have things reasonably stable and Nitrite is at zero, feed sparingly for the next week and keep a close eye on your readings. If Nitrite shows up, stop feeding until it drops back again. You need to slowly re-establish the bacteria colonies and this will take time.
Alan.
Posted on Jun 1, 2006, 6:01 PM from IP address 213.120.101.21