Now that you have plants, you don't want your nitrate levels at zero. In fact, you will need to add fertilizer, which will not impact your fish directly. Indirectly, it will affect them by having healthy plants, which will help keep your water clean and 'fresh' for them. So, study-up on macros (N-P-K) and micros (trace) here on TPT. You won't need much of each, but you will need some of each.
Tank capacity for stocking can be expanded by adding oxygen. Aerators don't add oxygen very well. Make sure your filter is creating good ripples on the water surface and/or add a small pump (I like Koralias) that, when pointed at the surface create wave-like rippling. You don't want to break the water's surface, just make it ripple well. This increases oxygen. It also drives of some CO2, which plants need, so you may want to add Excel, by Seachem, to help the plants get the carbon they need.
I have a greatly over-stocked 29-gal and have manged to get away with it for many years. Seachem also makes a good ammonia monitor (Ammonia alert) that attaches to your glass and is very inexpensive. you can then check your ammonia anytime by just looking at it. An increase in ammonia tells you you need to take steps to correct it, which may mean fish load reduction. Another indicator is fish behavior (swimming near the top when they normally wouldn't, little movement and aggressive behavior as they try to stake-out some territory of their own).