[KoiCop]

ob . . .

Like you, we use our Ultima II for mechanical filtration in front of the J-mat bio, and we flush it daily -- whether it's summer or winter, whether we're feeding or fasting.

Why? Organic waste that is flushed from the system won't decompose in the water column, so it won't produce ammonia and DOCs that then have to be dealt with later.

As for your question about water changes:

Starting out with your big pond, cold water, small fish and infrequent feedings I'd think about starting at 2.5% per week; come summer, maybe 5% per week. Then work upwards from there to 10% or 20% per week, or more, depending upon all the factors.

Water changes not only flush nitrates and DOCs to waste, they snap your water parameters (like pH, KH and GH) back to tap water levels and replenish necessary minerals which are used up by the fish and the filtration.