Well, lots of parasites live in lakes, as well as bad algaes, and probably a very different pH than what your well water will be. If you're going to use lake water, you're going to have to use it constantly. You also probably dont have water rights for the lake, so if you get caught taking water out of the lake for personal use you'll get fined big time.
I wish people would define 'treating' water. Just saying you 'treat' your water doesnt make it safe, and doesnt make it a cost effective practice. Dumping random chemicals into your water from the lfs doesn't make a lick of sense unless you know what the chemicals are supposed to do. Most chemical manufactures dont care what happens to you, your fish, or your wallet and will tell you anything they can that'll make you buy their 'mega ultra super safe water treatment plus one!' You dont have to treat well water, it doesnt have chlorine in it, and dumping dechlorinator into it will only cost you money and make it smell weird. Invest in a decent test kit, and test your well water yourself. It's just like any bottled water, untreated, non chlorinated, and totally safe for fish. It shouldnt have anything in it other than a ton of minerals and water.
I'm guessing your water will turn up high alkilinity and high pH.
EDIT: The bacteria we like that process ammonia are sessile, as in they stick to things. All you're going to get with a bucket full of lake water is a ton of non-sessile bacteria (opportunistic infection types as well), parasite larvae, and water. The nitifying bacteria we want are /everywhere/ floating in the air, covering surfaces, even in out bodies (some types of nitrifying bacteria can cause serious infections in humans, go figure) and if you add ammonia to your tank and be patient, they'll develop.
