Since tanks aren't expandable...
You can either find accessible space for a larger tank within 6-8' of the toilet. The space doesn't have to be rectangular, btw...we offer nearly 100 non-rectangular shapes to fit just about any space on a boat. Or-- there's an article in the Reference Library in the Head Mistress forum called "Flush With Success" that explains how to cut flush water usage enough to more than double the apparent capacity of your existing tank. You may find that's all you need to do.
