Has anyone used these holding tanks? I don't see anything about a vent. Wouldn't it create a vacuum making it harder on the eletric pump. Does anyone recommend a book on how to set up the plumbing in a boat? I pretty much have it all figured out, I'm a fireman so I understand PSI and GPM's; but I mathematical formulas do not always give you a good idea of what really works. How many gallons do you recommend for a weekend on the boat? What pumps do all you recommend. I have a raw water intake for a wash down so that is out of the formula. Here is another question: I have a Catalina 30 and the original freshwater tank is under the starboard berth. My last boat a Hunter 27, had 25 gallons up front under the v berth. What seems the to be the best layout? I have 2 batteries, a holding tank, and a fridge all on the port side. Will the boat perform better with weight foward from a freshwater tank?Sorry for all the questions but like most everyone there is only so much time and money and I am trying to keep both at a minimum by learning from all of you. THANKS !!