Get Rid of Pond Algae
What is all that green goo? Well, the long strands of green goo are string algae. The stuff making your pond's water into pea soup is plankton algae. Algae are millions of tiny little floating plants (single celled plants) that thrive when they have lots of sun and food to grow on. Like bacteria, there is no way to get rid of all the algae (unless you want a sterile, dead pond). The trick is bringing it back into balance.
So what can you do? Minimizing the light that the algae gets will help. Floating water lilies are a good start. Ideally you might have placed your pond in a place where it got partial shade for part of the day, but its probably too late for that now. Besides, you don't want to completely deprive your pond of light. A much better solution is to stop feeding it so much.
If you are adjusting the pH of your pond, you are unsuspectingly adding another essential ingredient for algae: phosphates. Most pH adjusters contain high levels of phosphates, a compound algae just loves. Switch your brand. There are pH adjusters that are phosphate free, but you may have to go online to find them. Pond Care pH Down, for example, is phosphate free.
Like all the recommendations in this article, just cutting off the phosphates won't completely fix your algae problem. You'll need to fight the pea soup on several fronts at once.
A lack of good bacteria - the kind that lives in biological filters - is another part of your problem. As the good or "beneficial" bacteria play their role in the nitrogen cycle, they consume the same nutrients that your algae does. So the healthier and more beneficial bacteria you have, the less green algae can live.
If you've got a serious algae problem, look to your biological filter, how many fish you have in the pond, how much muck is on the bottom of the pond, and how much water circulation there is. For starters, you're going to need to vacuum the pond to get out as much of the muck as possible - all that stuff sitting down there is algae food.
The next thing to address is your filter - if you really want to solve your algae problem, you're going to need a bigger home for those beneficial bacteria, and part of that means a bigger biological filter. You should also go buy some over the shelf beneficial bacteria. Microbe Lift is an excellent choice, but there are many others. Adding the new filter (or keeping the old filter and adding a second one) will probably also help with water circulation. Those beneficial bacteria need oxygen, just like your fish. By the way - one of the nastiest affects of an algae bloom is a big fish die off. The fish die because the algae has eaten up all the oxygen in the water.
With a better filter, you may be able to keep all the fish you had before, particularly if you're willing to clean the pond more often to suck up all the debris (aka algae food) on the bottom. Do keep in mind that every time you feed - or overfeed - your fish, you're tossing more algae food back into the water. Feed lightly. Hungry fish do sometimes chomp on algae.
And fish aren't the only ones who eat algae. Once you've gotten the algae problem under enough control, you can toss in a few snails or a few tadpoles to keep the algae at bay. These creatures are sometimes sold as a "cleaning crew" by pond centers or websites. They will help keep algae in check, but they can't eat enough to turn back a green fog.