Not sure whats wrong (Brown end leaves)

Well I have these 2 plants, one is sensi star the other is purple eurkel. The sensi star is doing just fine it seems but the purple eurkel keeps having the smaller leaves curl in brown and some of the bigger leaves the ends get brown as well. A lot of the other leaves are just fine though.

I use Bio-Grow and Bio-Bloom mix formula for fertilizer. They are in 5 gallon pots. Anyone have an idea why the plants turning brown on spots?

Also, in the last pic the one on the right is the sensi the one on the left is purple eurkel. But does the purple one look a little skinny to anyone else?

Thanks in advance!! :rastasmoke: