The fish have been back in for nearly 3 weeks now and been through 1 algae bloom (like thick pea soup) already which almost cleared and then we had a massive 24 hour long downpour on Sunday which resulted in the pond flooding and washing a load more peat back into the water again which apart from making it a dirty brown colour will most probably cause another bloom as soon as it gets sunny again.
........ you can almost guarantee that you will go through an algal bloom during the heat of the summer. That in itself is not a big deal, as algae is actually quite good for the koi. And it shows that the filter is doing its job - breaking extremely harmful ammonia down into slightly less harmful nitrates and then into the relatively harmless nitrates. (Which is, of course, where all that algae comes from...)
.......And the second way of removing nitrate from the water is to have a water course filled with plants - preferably water cress or some-such that devour nitrate!
However that and what you said above has got me thinking, I missed a golden opportunity by filling my marginal areas with peat, you can see these under construction in the next photos:
I am going to remove the peat, lay in some pre-drilled 40mm pipe all along the bottom and refill with gravel then re-plant those marginals and a whole load of water cress in there.
Then divert some of the flow from the waterfall - (which is built around the square 100 litre filter tank - photos to follow) - through the pipe so it flows upwards through the gravel bed.