Fredrick - The Nitrite levels are not raising as fast now. The last water change was probably over a week ago and today the level was .25mg/l.
There is nothing rotting in there. It's an ornamental pond with no plants. Just water, a pump and air stones.
I have notice though that the Titan 2000 pump collects solids underneath rather then send them all to the filter so I have got in the habit of sticking my pondovac under the pump weekly to get rid of any waste that is hanging around.
Otter - The strips were new. The problem is the scale. They don't read the low levels we wish to test. There needs to be 10mg/l before they start to show any colour change.
The Blagdon Minipond filter should be fine for a pond of that size. the built in UV is adequate, however, the Minipond pump isn't brilliant as it is not a waste handling pump. I would use a Hozelock Titan (actually I do).
Are you sure you have algae problems? Can you describe exactly what you see or even post pictures? A green tint can also be down to excess nutrients in the water. Take out a pint glass of water, if it looks totally clear, it's most likely the excess nutrients causing the green hue. A bacteria boost from Hydra Aqua clears that quickly (from experience). If there are very small particles, it may be down to the newness of the filter (at the time of writing, they have only been out a couple of months) as the foams are less efficient when new. They need to build up a little dirt before they are working at 100% efficiency. Again, Hydra Aqua do an extra fine electrostatic filter layer that you can use to boost the efficiency of the filter.
A pergola will help with the dreaded blanket weed. I have had no blanket weed since fitting mine. It will also keep Herons away. Not sure if it will help defend against a Heron that already knows of the pond's whereabouts though.
I would save the ?300 you have for a filter upgrade as I have found the Minipond to be a brilliant filter.
Sorry didn't post full results, but yes, you are right to assume that Ammonia and Nitrate are both zero. PH is 8.2.
I'm unable to move the fish elsewhere, so will monitor on a daily basis and carry out water changes as necessary.
I'm only feeding a tiny amount every couple of days. Water temp is down to 14C now, so they don't want a great amount anyway.
I double checked the test strips and noticed the scale. At the point they get even the slightest hint of colour there will be 10mg/l and a very light hue of purple 25g/l. Useless. The water would be highly toxic (as mine was) before you had a chance to notice - lesson learned in that respect.