Titus JC Ng Theseira asked:
I have an aquarium in my home and I was wondering, why the UV steriliser I have in it can get rid of algae, but UV in sunlight or from UV bulbs placed above the tank will actually paradoxically promote the growth of algae.
Helen - Well itís all about the types of UV (ultraviolet) radiation that you've got - both in the sterilising bulb that you've got in your setup, and thatís just all around us from the sun. [Sunlight] comes down through the atmosphere and contains UVA, UVB, and UVC. The most damaging form of UV is UVC, and thatís whatís in your sterilising bulb. It will be at a very high intensity and your water will probably get sucked through a filter and fed past this UV bulb, and that will do a very good job of killing off all those bugs that you don't want, using this very powerful form of UV light. But the UV light thatís around us naturally from the sun, or even from a light bulb above your tank, that UVC will be very easily blocked by the water. Because it has such a short wavelength, it actually gets very easily disrupted and it won't actually make its way very far into the water. Thatís why nothing in the tank really gets affected by it. So you need that very concentrated intensive burst of damaging UV to keep the water clean, but otherwise, we do fine with the UV that bounces around in the room.
Chris - Because the stuff that weíre normally seeing is a little bit of UVA, a bit of UVB, and we have an ozone layer which is really good at excluding most of the UV, including that UVC. So, in terms of sunlight, for the most part, weíre protected.
Helen - Mostly protected. We still can get burned by A and B and different sun creams can help to block that, but in terms of the life thatís in your tank, I think the water is enough to keep them safe from any bombarding UV light thatís coming in from the surface.
Chris - Most filter systems have a little bioreactor in the bottom as well, where you have lots of bits of plastic and things with a high surface area. Good bugs, ones that you like, grow on there and then after the waterís gone through the steriliser - which wipes everything out including the algal cells - it then goes through this bioreactor in the bottom of the filter which then re-seeds good bugs back into the water, as well as consuming some of the organic content which is how these filters actually work.
Helen - Absolutely.