They agree that your problem is most likely due to overtightening. Step one: Call 'em 800-544-2132 and buy their rubber washers...that guarantees you have the right size and thickness. When you get the right washer, wrap the bolt threads in teflon tape and tighten only till it feels snug...don't keep tightening till it's down as far as it'll go. There's a "rule" that applies to plastic tank fittings in plastic tanks: wrap in teflon tape and tighten only ONE full turn past hand tight. If it leaks, try just a half turn more. It should seal, and you won't break any more bases.

If you still have questions, try picking up the phone instead of emailing...emailed questions often require replies and additional exchanges that are too time consuming, so they go unanswered. Ask for Doug and tell him I sent you.