All of your problems are caused by lack of nutrients. In addition light and co2 and water plants need nitrogen, potassium, Calcium, magnesium, phosphate, sulfur, Chloride, iron, manganese, boron, zinc, copper, molybdenum, and nickel. If just one if missing plants will not grow. and could possibly die. Now tap water willl have some nutrients but not all. And with one water change per month you are likely running out of few nutrients you water has. So more frequent water changes would help. But you also need to add a fertilizer. But with one fertilizer dose per month you are likely going to run out of nutrients well before then next water change And that could lead to algae. Additionally most fertilizers on the market depend on your tap water supplying some of the nutrients. So you have to address both the water change and fertilizer problems.
I haven't seen any posts of successful agauriums with maintenance only once a month. But that said it impossible to systems to do automatic water changes and fertilizer dosing. but that is going to take some skill with tools and time to setup and get it working correctly.