Eco-complete doesn't lower the pH and is quite nice, I use it in my planted 55 gallon. You can also use flourite as the cap, which looks quite nice, but I feel like my plants don't root very well in it. It does however over time increase the tds, mine And gravel should be fine, I heard people use it often and its cheaper than buying something else, never tried it myself, but if I do I might with a under gravel filter to see what happens as the ferts are pulled straight to the roots. By the way I also used the walstad method and I love the results, it works better with dosing and CO2, but will still grow fine without dosing. It is easy to kill shrimp with nitrogen fertilizers, so don't dose too heavily in the tank. I read algae blooms might occur due to the amounts of nutrients in the soil, if you don't plant heavily in the beginning. My method to stop algae from forming during the beginning phase of planting is floating plants. Good luck!!