The Nexus, or any other filtration system, cannot begin to work until you have some fish in the system. It needs the waste from the fish to start the bacteria.
It would appear from your description that you do not have a heater. In that case there is no point in adding fish until the pond water is at least 50 degrees F, and the fish start eating.
I would suggest you shut everything down, and empty the pond. If you do this then make sure all the pipework and components are completely drained so that it cannt freeze and cause problems.
You can then start again in the Spring when the temperature is much warmer and you are able to buy fish to go in an outside pond.
A sand filter will only polish the water, and will do nothing about green water.
It sounds like you have built a first class system and it should work well when we have warmer weather.
Posted on Feb 23, 2006, 6:34 AM from IP address 86.128.93.93