Hello
The weak stems could be caused by many things happening, my most common stem problems was from a calcium deficiency...that actually was often caused by my ph troubles. The best thing you could do for your indoor grow would be to just buy a decent ph pen and the liquids that adjust it. You can go about it in many different ways, lemon juice, baking soda etc. but the drops will be easiest and most accurate. Given the 2-2-2 nutrients i think that most the things you need are there, but unavailable.