By Spike Cover
This is a good explanation and drawing of how to build a constant flow regulator for doing constant water changes on your pond.
Many folks have struck on the idea of using a slow trickle of fresh water into their ponds paired with a standpipe-type overflow to help rid the pond water of nitrates. The simplest and cheapest way to accomplish this (presuming there is a suitable overflow) is to use off-the-shelf drip irrigation components to regulate the inflow of water. However, knowing that these components are less than precise and prone to drift (varying the flow rate) and other forms of failure, I decided I could design a low-flow device that would be pressure compensated, could be precisely set and likely more reliable than store bought drip irrigation parts. (In a former life, I was a mechanical engineer with a background in hydraulics.)