Ronco Plastics makes all the Catalina OEM tanks
They have for years. However, that doesn't necessarily mean that Catalina's choice of size or location is the best one. Their approach to holding tank venting (into a rail stanchion that has a tiny slit in it) stinks--literally!There really is no "one size fits all" answer to your question, 'cuz a previous owner may have installed a transducer or a knot log in the best place for a tank, making it impossible to put a tank there. So the best advice I can give you is, look for a space that offers the most direct plumbing run from the head to the tank, with the fewest "over/under/arounds," within 6-8' of the toilet. Then give Ronco a call...not only do they make tanks for Catalina, but they have more than 400 shapes and sizes--including over 100 that are non-rectangular...they make TOP quality tanks, and you can spec the fittings where you need for 'em to be instead of trying to work with "standard" off-the-shelf locations. Ronco has a website: http://www.ronco.com but less than half their tanks are on it. If you can't find what you need there, give 'em a call for a full catalog. Once you've explored the possibilities on your boat, I'll be glad to help you work out the detailed specs one-on-one if you'd like to e-mail me. I'm in all the owner directories for all the sites that share this forum.Btw...I don't own a Catalina...I don't even own a boat any more <sob> since I sold mine in January. I just happened to make a very unusual career choice about 15 years ago that has given me the knowledge and experience to know what will and won't work in all situations.