I wouldn't bother with a UV steriliser on a home setup/ recirculating system. If you quarantine all new fish, plants, shrimp & snails for a month, and don't introduce water from pet shops or other people's tanks, there should be no need for a UV steriliser.
You should also treat any fish you have for intestinal worms (tapeworm, thread/ round worm) and gill flukes. All the tanks should be done at the same time. Praziquantel is used for gill flukes and tapeworm, Levamisole is used for thread/ round worms.
Once your tanks are set up and the fish are free of disease, the only way new diseases will get into your tanks is if you introduce them (eg: white spot, velvet) or let the tank conditions deteriorate to such a level the microscopic organisms (bacteria, fungus, protozoans) build up and overwhelm the fish. Regular water changes and gravel cleaning, and filter maintenance will prevent disease outbreaks.
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If you do have a UV unit on the system it should be on continuously (24/7) to work most effectively. If you have a UV steriliser on for a few hours each day, the unit will kill microscopic organisms when water is passing through the light, but it will do nothing to disease organisms when the UV light is off. If you plan on using one, you want all the aquarium water to go through it all the time.
You want the water to slowly pass through the unit. The more time the water is exposed to the ultra violet light, the more microscopic organisms that are killed off. UV sterilisers normally have a recommended flow rate that suggests x amount of litres per hour pass through the unit to kill 99% of harmful organisms. Most units do not actually kill 99% of microscopic life at this flow rate. If you can reduce the rate the water flows through the system (to less than recommended) they do a better job.
You need to have the UV unit on the outlet off the filter so the water passing through the UV unit is as clean as possible.
Most UV units can be housed either vertically or horizontally. Some brands say they have to be kept on their side, in which case follow those directions. But if there is nothing on the packing about being installed vertically or horizontally, then it won't matter for that brand.
When you do maintenance on the filter, you should turn the UV unit off about 5-10 minutes before you let any water drain out of it. This allows the UV globes to cool down before it is emptied.
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Even if you do have a UV steriliser on a system, there is no guarantee it will kill all the disease organisms in the tank. Microscopic organisms can live in aquariums and not get washed into the filter. And the UV unit will only kill the organisms passing through the UV light.
Fish that are not exposed to some microscopic organisms (in low levels) can become weaker from a disease resistance point of view. Years ago when I was breeding rainbowfish and selling them to shops, one of the shops said they were losing a species of fish I supplied them with. They asked if I was using a UV unit and I said no. But the fish they were buying were getting sick and developing diseases within a few days of going into the shop tanks. It turned out my tanks were virtually disease free due to massive daily water changes I was doing and the fish did not have a strong immune system because the fry had not been exposed to any of the common diseases found in shop tanks, and were contracting diseases when they were introduced to the shop tanks. The shop eventually set up a new tank with clean water, gravel and plants and the fish were fine in that.
In a shop that I worked at, we had 100+ tanks on a shared sump. Each tank had a corner sponge filter, a thin layer of gravel and an airstone. All the water from all the tanks went into one sump and was filtered and pumped back into the tanks. We had new fish coming in each week and sometimes they brought in diseases. If fish got sick on the system, we would turn the tap off from the recirculating system and let the corner sponge filter keep the tank clean while we treated it. The diseases never spread to other tanks even if we didn't isolate the sick tank immediately by turning off the tap.
I see no reason to use a UV steriliser on a home aquarium. Basically if you have a UV steriliser on a shared/ recirculating system and in a home set up, it is unlikely to make any difference to disease outbreaks. Just quarantine all new fish, plants, etc, for a month before adding them to the display tanks. Treat the new fish for worms and flukes while they are in quarantine, and if they are healthy after a month, then add them to your display tanks.
Save your money and buy a video camera to film your fish
