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.

View attachment 1123656View attachment 1123657View attachment 1123658

Any advice would be really appreciated. Thanks!

Looks like an N deficiency

really? I thought that I shouldn't be giving much nitrogen now that they're in flowering stage.

Not nitrogen. Is it starting on the bottom leaves. Looks like my magnesium deficiency.

canadagirl said:

really? I thought that I shouldn't be giving much nitrogen now that they're in flowering stage.

Click to expand...

Foliage is what is needed during flowering, N supports foliage. Yellowing leaves from the bottom up is a N deficiency. You might want to switch to a fertilizer with a higher N value.

It's not nitrogen. Nitrogen doesn't cause the leaves to dry and curl up like that. Also if you notice, only 1 of those leaves is really yellow like N deficiency. Read this. http://www.icmag.com/modules/Tutorials/PlantTrouble/Merged.htm

I guess the guide says it's mg. That makes sense cause we have really hard water. I'll give the epsom salts a try

Ok so I finally found some epsom salts, I didn't realize that they could be found at any drug store....
Does anybody know whether its a good idea to foliar feed them when they are 10 days into flowering? Or should I just add it in with watering?

I believe 10 days should still be ok. I read that you should stop foliar feeding the 2nd week. Not really sure though, the nutes I used just had mg in it.