That Anubias' new leaf looks more like A. coffeefolia than nana, very nice! Usually when my plants get holes they are hungry for something or another, not sure which in this case. The green spots are just a bit of GSA. Just the right amount of phosphorus keeps it off my Anubias but that isn't enough to worry about. My Anubias do great in bright light and currently there are several flowers on them.
If GH/KH are higher than you would like you can just mix the RO in the proportions that would give you the water you want in the tank. If the tap is 12GH and you want 6 then mix half and half for instance.
For a while my substrate was mostly very light weight Schultz Aquatic Soil, I just hold the siphon over the substrate, mulm and such are lighter weight than the SAS and float up and away. Any cap is going to get mixed into the rest of the substrate unless you work really hard to keep them separate.
The nutrient calculator here will help. Seachem liquids are listed. http://calc.petalphile.com/
Most people get annoyed with HOB filters after a while. I think they are noisier and have less filter media capacity than canisters? I like that they are easy to get to to rinse out the media and keep the water surface clean with a nice ripple. Just keep the water level high to avoid outgassing CO2 with a lot of splashing.