UPDATE: (almost 2 weeks on from the problem starting)
- started with an initial dose of Seachem Pond prime to bring down suspected high nitrogen levels (did not test at that time as no test kit)
- 3 separate 20% ish water changes every few days for first week, refilled slowly through a dechlorinater
- doing liquid tests (API 500 kit) every few days to check PH, nitrite, nitrate and phosphate levels are safe
- feeding a small amount and making sure it all gets eaten so minimal waste food in water
- cleaned the pressurised filter using the auto clean feature today as the water starting to look a little green, I did not want to do this too early as trying to establish the correct bacteria to deal with the nitrogen cycle. The water that came out of filter waste pipe was very dirty but ran clean after 90 seconds. I will be testing again this evening to check no adverse affects from the filter clean.
Liquid test results show the PH level goes alkaline and towards 9ish when water has been topped up but comes back down to 7 ish within 2 days. Ammonia and Nitrite levels very low, maybe a trace. There is a small amount of phosphate showing on the tests now.
I have lost 3 fish in total, 2 of which were the ones showing clear signs of distress at the start. The rest seem a lot happier, swimming and floating in the water normally. Hopefully I now understand enough to keep the water well balanced for them. I now just need to address the overstocking issue as they grow.
Thanks to all contributors.... very pleased to have this free resource to call upon. Great work