What kind of deficiency is this? Pics

I have 2 Hawaiian snow plants 1 week into budding. They're about 4 feet tall, but tied down. I haven't given much nutes because I was advised not to by a long time grower, but I think they are too big for the amount of soil they are in to not be getting any. I was feeding a 20-20-20 mixture every week and a half, not very strong.
They're big plants and definitely root bound, but I just don't have the space to transplant them into bigger pots. I'm trying to salvage them as much as I can and just learn from my mistakes for my second grow (i.e. bud them a lot earlier)

The water here is really hard, so that might have something to do with it.
Some leaves are yellowing. A few leaves (4-5) on each plant are completely yellow with brown spots. Near the base of the plant some leaves have completely shrivelled. There still are many healthy leaves, but the very tips of them seem to be slightly crispy, and the yellowing starts from the tips or edges. New growth looks healthy and is bright green with no sign of yellowing.
I tried looking up what it is, but I just can't pinpoint it. I think someone on here would have a way better guess about what it is than me. The most recent watering I used distilled, in case it was related to the hard water here.

Please have a look at the pics, and take a look at the tips of the healthiest looking leaf - that's how it starts.




Any advice would be greatly appreciated. Thanks!

your ph is probably off from the water. i would give it the distilled water till it corrects itself. could be a mg deficiency

any second opinions? Does everybody think that this problem will go away as long as I keep giving distilled water?

check the ph in your water. if its around 7 then your good. If your feeding flower nutes, you might have a nitrogen defec. 1st check ph

pH or magnesium deficiency.

Flush and see if it improves which I feel it will.