OK, lets say you have some nice barrel filters going and all of a sudden your water turns green. Here's a simple adapter you can make to fit the outlet of just about any kind of barrel filter to take the new 50 watt Savio quartz less UV sterilizer. In this example I have it set up coming off the discharge of my sand/gravel filter.
Just take a 3 inch "T", PVC or ABS, put a 3 by 2 inch bushing in the top, then a 2 by 3/4 inch bushing we have to modify. The 2 by 3/4 inch bushing has a taper to the bottom that is the same taper as the Savio UV base, (see pictures) So we want to glue this bushing in the 3 by 2 bushing up side down so you will have to sand it down some to get it to fit.
Then cut a piece of 3/4 sc 40 PVC 1 inch long and glue it in the bushing. This makes for a nice fit for the UV unit. Then glue the 2 by 3/4 bushing into the 3 by 2 bushing.