Ditto what was said above. UV is highly corrosive and will break plastic down over time. The cheapo ones will only last for so long so they're better for specific treatment rather than 24/7/365 use. On my old 50g tank, the green machine didn't do squat against dinos, but a 15w AquaUV with a properly sized pump eradicated them. I like that the higher end units have published flow rates to exposure (microwatts per square centimeter, µW/cm2) so you can dial it in and be effective against what you're targeting.