leaves turning yellow brown and dying please help

The leaves on one of my plants are turning yellow and brown and dying. I have had this problem before and the leaves have always died before the plants were ready. I have 8 other plants and they all look fine. I am using ocean forest soil with the 3 fox farm nutes, but I don't think I have been over feeding. I am not sure what the ph of my soil is or how to even test it but I only used distilled water. Does anyone know what this could be? Thanks for the help. I really don't want to lose this plant.

looks like a calcium mag deficiency...distilled water doesn't have any...calmag added to water will fix that. Here is a chart I have found useful.

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Before you planted em in the FFOF did you add anything to it, like lime or anything. I know for myself I always like to add dolomite lime to my OF soil before I even plant anything in em. In part to lower the ph cause the OF soil is usually around 7, and I know from when I was looking into using the OF stuff a lot of other growers were sayin that the OF stuff didn't have enuf of like the oyster shells and other calcium stuff. And Pixi is right about using the distilled water, is ur tap water there really that bad. One guy on here said when he grows in soil he uses his well water and his plants love it, u would really only need to go so far as to use distilled water if u were doin a hydro grow even then a lot say they don't bother. But from the pic idk if it looks like a calmag def to me the way the tip of the leaves r curling up seems more like burn to me.

I did not add anything to the soil before i planted. my tap water is fine i just figured distilled was better for some reason, that will save me a trip to the store every other day. I also have irrigation water i use for my outside garden i could use if you think that would be better. Also what is the best soil to use ? thanks for the comments