Here's my thoughts and advice-
First, what you have is working...no need to freak out...but it's not ideal IMO. When I first suggested spring water, it was a Hail Mary just to see if we could get things moving...turns out it did. Now when I first suggested spring water, it was strictly thinking about cycling...not stocking.
The Poland Spring you have in there now looks good. Problem is it's not consistent. You said when you tested it before it was the low 6's...now it tests at 7.4, so there's definite variations bottle to bottle. It's not really a problem, and fish can take small swings during water changes without much harm...but again, it's not ideal. Remember this is NOT about fish adjusting to your stable pH level...it is the challenge of providing a stable pH at all.
* Ideal Solution IMO- Pretend you have a saltwater tank. Start with pure water (no minerals, no chemicals) and make the water to the exact pH / kH / GH for the CPD's by using the additives designed for freshwater, purified water. There is no issue with doing this...in fact it provides perfect water for your fish. The reason you don't see us recommending this to people is because tap water works for most fish, unless it is in extreme ranges...so there's not normally a need to do this. Basically it idiot-proofs things and saves people some money by advising this. Since something is screwy with your tap water...IMO, this is one of those situations where you should break away from the norm. It also gives you a rare chance to provide absolutely perfect, ideal, pristine conditions for your CPD's.
If you did that, you've got to start with pure water. That can mean buying your own RO system, buying it in bottles or buying it from a LFS that sells it. Start with pure water, research ideal conditions for CPD's, experiment with the additives, and use it for every water change. That way you will provide the exact same water that's already in the tank every time you add new. I'm sure pure RO water (without minerals added back...so it will always be pure with the same pH) can be found at stores...probably a no-name generic brand at Walmart or Target, etc... I've never researched starting with distilled water, so I can't advise it...but in theory it should be perfect. Problem there is distilled usually costs more because the purification method is more intense.
I know it seems a bit overwhelming...just remember that every member here on the SW side does this every day. So......my recommendation summed up....start with pure water, add the RO replenishing minerals to make the ideal water conditions, enjoy your fish.