Composts and fertilisers applied to the surrounding garden are high in nitrogen and can cause algae problems. Also fish foods are high in nitrogen, so make sure you are not over feeding your fish if you get murky water.
As with algae, green pond water and blanket weed can occur in new ponds when most of the water has come from the tap and is high in nutrients like phosphates and nitrates.
Blanket Weed is a problem nearly all pond owners encounter at one time or another. Blanket Weed can quickly fill an entire pond during the summer months.
Please note, barley straw bales will have no effect on blanket weed.
One of the first things you should do is increase the oxygen content of your pond by introducing plants like Elodea Canadensis during April or early May which can help to compensate for the oxygen lost during decomposition of debris in pond water during the winter.
The decomposition of the correct volume of barley straw creates humic substances that will inhibit the growth of algae. For additional information click here.
Consider fitting a net if there are a lot of leaves getting in to your pond and take care to remove the leaves at regular intervals especially during autumn. You should also consider fitting a fountain or waterfall to your pond, this will help to put oxygen back into the water.
If your pond already has a pump and filter the easiest way to get clear water is to add an ultraviolet clarifier. As the water passes over the UV light the algae is killed. Once these are added to a filtration system the water normally clears within a few weeks and then stays clear all year round. These units start around £50.00 and cost around £10.00-£20.00 a year to run depending on the size of your pond.
If your pond does not have a filter, a good growth of plants especially lilies & floating plants is an excellent way to keep the water clear. These provide shade and compete with the algae for food. Aim to provide 60% or greater coverage to minimize light availability.
An additional help to control the growth of algae is to add Ramshorn snails to your pond, although if you have very large fish they may eat the snails. Tadpoles are very effective algae-consumers so do not remove any frog or toad spawn from your pond - they will produce thousands of little scavengers. There is also a species of fish that eats blanket weed. These are called Grass Carp although they can only eat relatively small amount and will not discriminate against algae & plants.
Some people add one pint (568ml) of 3% hydrogen peroxide per 1000 gallons (4546 litres) of pond water. This is said to have no significant impact on plants or fish, and is reported to work well on both suspended and string algae. The only potential problem is that if you have massive amounts of string algae you may create a very large oxygen demand. So physically remove as much as possible before trying this method - this caution cannot be over emphasized. Adding hydrogen peroxide, while effective in removing algae, will essentially result in re-dissolving the nutrients which contributed to the algae growth in the first place.
You may have seen the electronic blanket weed controllers advertised, we have used these and have noticed little if any improvement.
You can obviously remove Blanket Weed manually, although this can be almost a daily task.