Algae is usually present in any planted aquarium. A little algae isn't a bad thing. The way I determine what amount of algae is correct, is my personal preference. Some algae on driftwood and rocks make the aquarium look more natural and are a food source for some fish. The best thing to do is I.D. what type of algae you have. You said green algae, but that can be "green water"(floating algae) green spot algae (hard green spots on the glass) hair algae ("fuzzy" green appearance, slimy to the touch)etc. My findings is that the usual culprit is Phosphates. Aquatic plants use/need very little phosphates compared to nitrate and potassium. Most if not all fish food has phosphate added because it is a necessary nutrient for fish. Some foods have more then others, so look at the ingredients. Stick with the higher quality ones. It depends on how far you want to go to solve the problem. You can purchase testkits that show Nitrate and Phosphate levels, then address which one is the problem. It gets tricky though, sometimes the source of excess nutrient is from the tap water. Nitrates can be the trouble also. Potassium is really not an issue with aquatic plants. They really use the stuff up and from what I've researched they can't get enough of it. If you got a test kit for Potassium, and tested your water, I bet it would be under reccomended levels/ratio. There are safe products to remove Phosphates like Phos-Guard. There is a slightly less expensive one that comes in packets that fit in any filter. I think PetSmart sells them. The thing to remember with these products is use the Aluminum Oxide ones (little white beads). They only remove Phosphates and Silicates (they cause brown algae or diatoms) and don't release anything into the water. As far as lighting the proper ammount of light is 12-14 hours a day for a planted tank. The ammount of light should be 1.5-2 fluorescent watts per gallon (WPG) for low light plants, 2-3 WPG moderate light plants, and 3-4+ WPG High Light plants. If you have too much light on low light plants they don't use enough and the algae has plenty of "food". If you have too little light on high light plants, they don't grow well or fast enough and now the algae has food from the excess nutrients the plant isn't using. If you go over 2.5 WPG you should consider adding a source of organic carbon (CO2). Even if you are staying under 2.5 WPG adding a liquid supplement like Flourish Excell will really benefit your plants. Be careful of dosage if you keep anacharis/elodea, use the minimum or less then minimum treatment. Finally, about 20+ Oto cats for fish. These are very small, efficient algae eaters. They don't get bigger then 3/4" and don't do much to your bio-load. As a matter of fact they really don't effect it at all and personally I don't add them to the total when equating my fish stock. They are small enough to get on the most delicate plant leaf and suck of any algae without damaging it. They really do a great job and last a long time.

If blue algae is rampant, then there is a chemical imbalance as a result of more nitrates being added to the area (say fertilizer in rain run offs, or laundry soap, sewage, etc..). Check with your local environmental group to see what they suggest. Don't do any DIY solutions otherwise you'd risk introducing a possible invasive plant.

The big concern with outside water is geardia and other parisites that live in water as a host. The alage will not hurt the cat. These bugs cause diarea and vomiting, fever and listlessnes. I do work for a vet and we see these things all the time in the summer months. Pond WATER my make them sick not the alage.

Hi My new pond went green with blanketweed in the spring. I removed some by wrapping it round a large stick, but once the plants started to put on growth the water turned brown. It only stayed brown for around a week and then went crystal clear and I've had no further problems. I also don't have any filters etc. It may be that you just need to be patient for a little while and nature will sort it out for you. Cheers, Paula