I currently have a fairly new planted 75g tank and everything seems to be fine except that my plants are basically neither growing or dying. I'm looking for any suggestions as to what might improve their growth and propagation.
is right about the macros, but I’d expand the view to consider light and what you might not think of as fertilizer as you go about the process. As a beginner, you might want to invest in the planted tank guide offered on this forum.
You are not injecting CO2, which makes your tank what is called a ‘low-tech’ setup. This means that you should keep an eye on your light. Learn about PAR and PUR values. Start with this link:
Without CO2 injection, you should limit your PAR to no more than about 50 at the substrate and I would also dial the photoperiod down to about 6 hours. Too much light, without high CO2, can cause your plants to stall and weaken. Then algae can flourish. Products such as Flourish Excel or Nilocg Enhance are particularly good for low-tech setups to help supplement carbon. However, they can damage a few plants and Vals are among those so, avoid these for now.
Regarding fertilizer: in addition to the standard macros (N, P and K), think about calcium and magnesium. These are reflected in your GH readings. Common recommendations are between 4:1 and 2:1 Ca:Mg. A GH number, however, doesn’t tell you how much of each you have. For example, although unlikely, your 3 dGH might be all Ca. To be on the safe side, I would add about 2 dGH worth of GH booster-type products, such as Seachem’s Equilibrium.
There are two basic camps on fertilizing: PPS and EI (both can be studied on this forum and, I think, are explained in the guide). I am inclining more and more toward PPS, but EI is also good, particularly for beginners in high-tech setups. For low-tech, PPS is better for beginners …IMO.