Do a pH test at dawn and another pH test in the late afternoon. If there is a drastic difference between the two pH tests then there is a good chance the algae is causing the diurnal swings. The diurnal swings are often because of the algae using up all the dissolved carbon dioxide during the day but at night it will use up dissolved oxygen and give off carbon dioxide. Carbon dioxide dissolved in water is an acid and will affect the system pH. If you are having algae like that in your fish tank, you may not want to just leave it. I generally recommend shading the tank to nearly black out and make sure you have plenty of additional aeration since algae can rob your system of dissolved oxygen during the dark hours. Algae can also rob the system of dissolved oxygen when it dies and decomposes.
TCLynx - WINNING!
thanks once again, sensei
I like to use dark fabric so it can still breath and it can still get the benefit of some heating at the same time.
I've got fencing with netting and several layers of shade cloth over my big tank. The 300 gallon now has a little structure over it with a black curtain on the south side but just netting around the other three sides.
UV works too, if you happen to have the equipment (and I would probably recommend the UV clarifier to people using a swimming pool or ornamental water feature that can't be covered up.)
Now if you are doing some media bed type aquaponics, once you get the algae to die off and start decomposing down in your gravel, you have a secret weapon to help inhibit algae for a time. As the algae decomposes it gives off chemicals that do inhibit algae growth. I believe it is some of the same or similar chemicals that are given off as barley straw decomposes (this is why barley straw is so often used to try to naturally control algae in ornamental ponds.)
However, if heavy shading of the fish tank is possible, it will by far be the cheapest method to control algae.
Thanks for the good info guys, I am going to try the shading route for now as it is a little cheaper and I think it will be fairly simple to do.
Nate I looked up tannic water and I am not sure what it does specifically? is it for killing algae or for lowering of the pH?