I would not even think about doing a 50% water change frankly. You fish won't mind green water whereas a 50% water change could well cause them harm and your filters. Mature water is valuable. Remember that! If you have hard tap water with lots of phosphates in it, you could make the problem even worse.
Give the pond a chance to settle down for a couple of weeks before you do anything drastic. You have killed off LOTS of blanket weed and the filters have reacted by converting all the extra ammonia into Nitrate which exasperated the green algae. Try and kill that and the same problem will occur.
Do things in this order and you should resolve the situation:-
1. Make sure your UV quartz tube is clean and the bulb rated at 10W per 1000 gallons.
2. Make sure you have a turnover of the entire pond volume every 2 hours or more.
3. Make sure your filters are properly removing fine sediment and debris in the mechanical stages.
4. Make sure you have plenty of O2 in the water column. Add air if necessary with a pump.
5. Stop using the products you are using and let things settle down.
6. Check your Nitrate & Phosphate levels and reduce feeding to reduce these levels if necessary.
In a couple of weeks, you can remove as much blanket weed as possible by hand and try something new. Blanc Kit seems to work quite well. Let the UV cure the algae problem. You shouldn't need chemicals to sort out green water of your UV is big eenough, your filters can trap the clumped cells and your turnover is sufficient.
Patients is the order of the day with green water.
Alan F
Posted on Jun 28, 2006, 1:40 PM from IP address 213.120.101.21