Any fertilizer will feed algae as will fish manure and snail droppings. There is one product for water lilies that is a slow release fertilizer spike. The advantage of this is that a larger percentage of the nutrients reach the plant and a smaller percentage dissolves into the water. For this reason, much smaller amounts of fertilizer can be used making the whole thing a much better situation.

I only fertilize my lilies and lotus. I never fertilize any of the other plants. The reason they are there is to scavenge nutrients out of the water to prevent algae.The Once-A-Year fertilizer tabs are by AgSafe. Someone in this group used them last year and like them. I couldn't find them in local pond shops so I ordered them from:

http://www.pondsplantsandmore.com
ponds@pondsplantsandmore.com
724-239-6673

They list them as:
AgriTab: Aquatic-Spikes Plant Food (8/pk Bag) $5.95

They recommend 2 spikes per lily. Once-A-Year Aquatic-Spikes from AgSafe (a div. of AgriTab Corp., Clearfield UT). They contain: Ureaform, Amoniam Phosphate, Potassium Nitrate, Potassium Sulfate, Calcium Sulfate, Ferrous Sulfate, Iron Sucrate, Magnesium Sucrate, Magnesium Sulfate, Manganese Sucrate, Manganese Sulfate, Zinc Sucrate, and Zinc Sulfate in a time release format. The are safe for fish and other aquatic life. The nitrogen in their formulation is 70% ammonium based and 30% nitrate based.

I used two spikes this year since it was the first year I have fertilized. Next year I am just going to use one spike. I got a small algae bloom when I put them in, but it is coming under control.

Here is a link that might be useful: Once-A-Year Aquatic Plant Spikes

I use commercial lily tabs. What ever you decide to use - push it way down in the pot near the lily roots and cover back up. My lilies are planted in solid containers so leeching is minimal. I use two tabs per lily container and that is it for the summer. If I use more my waterlily plants get too big for my pond and the pads start stacking up. I use the same tabs on all my non waterlily plants too.

Some folks swear by Jobe's tomato spikes. Everyone has their preference.

Phosphorus in the water will screw up your water quality, but nitrates, potash, and the micro-nutrients won't. Make sure any fertilizer that contains phosphorus is only put into non-permeable soil, like clay. Otherwise, it ends up in the water, and that's bad.

You can also use regular, non-pond timed-release fertilizer, such as Osmocote. Best to do when you repot in the spring, so you can mix it in the bottom layer of the soil in a holeless pot. That way it doesn't create an algae bloom. That won't help you this year, but it's an easy, cheap and effective way beginning next year.

I use Jobe's tomato spikes because they are cheaper than pond tabs. I only feed my waterlilies. All other plants are in gravel in mesh baskets so they can absorb nutrients right from the water.

I use the Jobe's spikes too. They are much cheaper than anything sold for pond use and they work fine. They are easy to push into the pots.

Another vote for the Jobe's tomato spikes from me :-)

I've been using the Job flower spikes for 2 years, all's well so far - use them in the lillies and all the isiand plants whch are mostly land plants. Do not use any soil, just poly batting and pebles./rocks.