Ok I still cant really tell what is going on with the leaf in the pic, I think maybe a lack of water issue.... But Im really not sure... Ive taken your pic and Im gonna post it in this post so others can see it well and hopefully we will get you the help ya need that way.
if the bottom leaves are the only ones that look like that I really wouldn't worry about it. Is it like that on any of the new growth up top or the leaves above the leaf in the pic?
I say that because some of the bottom leaves on my plants look a little like that but the plants seem to be doing fine.
regarding nutrient solution, I have a digital PH meter (tests the acidity or basicity of nutrient solution) and a digital PPM meter (tests the parts per million of nutrients in the solution) and they both make my life very easy. If you don't plan on getting the meters, they do have a very cheap PH kit for around $15 and I believe they have them at aquarium shops. They come with the PH acid & base liquids and a PH test kit that goes by the color of the liquid. I've heard they aren't that hard to use and they're much cheaper than the meter. They say the optimal PH of your nutrient solution is between 5.5 to 6.5. I keep my PH around 5.7 to 5.9 and that seems to work good for me. As far as PPM, I don't know a way to check your PPM without the meter and knowing the exact PPM of your solution is kinda important. Simply going by the required amount of nutrient solution that the bottle says to add always ends up with a different PPM (for me anyways). example: my flora nova bottle says 2 teaspoons of nutrient solution will amount to 950 PPM. When I use exactly 2 teaspoons of my nutrients the solution PPM is around 1100. I'm sure they're are plenty of people mixing their solution without meters but I just know it makes it alot easier for a beginner like me. keep in mind that I'm new at this and I'm just going by what I've learned here and my personal experience of my first grow which I'm still in the middle of...lol