What is the average depth of your pond? What brand of pond dye are you using? How much dye have you applied and how often are you re-applying it? Have you had similar problems in past years? Is there a known nutrient source that is getting into your pond like lawn fertilizers, ag fertilizer or possible septic leak? It is possible to have a water test done and to find out what your nutrient levels are like. If the test indicates your phosphorous levels are high there are a couple of products on the market that can be applied that will bind to the phosphorous and make it unusable for the algae and pondweeds but if you are getting a constant flow of nutrients every time you get rain it may not give you much long term control. How long ago were the grass carp stocked? You can use several products for control of the sago pondweed. My personal favorite has been aquathol K (liquid or pellets) but weedtrine and Reward should also work fine.