Well,a little of the osmocote goes a long way so maybe a cup for your tank sprinkled on the bottom glass before adding substrate would be my suggestion.
OK, thanks for the quantitative answer, as that's one concern I had, was how much. I'll probably err on the low side anyway - it's easier to add later than remove.
Problem I see with your plan is the black diamond being smaller grain,will find it's way to the bottom and the eco-complete being smaller grain than the gravel will do the same.
Maybe just use eco-complete and cover it with the pebbles? Or..
For your size tank,I might be tempted to just place the sprinkling of osmocote on the glass and cover it with black diamond and be done.
Eco complete would be expensive for this size tank which is I believe why you are also wanting to use pebbles and the black diamond to help reduce how much is needed?
Well, was thinking both of cost (it's almost the cost of the tank to use all ecco-complete!), but also was trying to get a variety of sizes and packing characteristics. Ecco-complete mounds up better than the Black Diamond also. But with varying size grains in the substate, I figure plants will figure out where their roots work best.
But I keep reading good things (in fact just see a post from Topekoms also) about it solely. Maybe will do just that plus some pebbles for appearance. While it may change over time, I could just pile pebbles in one corner or something to make them stand out.
But i need something for height... I don't think the Black Diamond is going to mound up at all, is it? What do you do with it if you want some height in places? Even if I put something under it (say starbord or acrylic solid sheets) I assume over time it just runs down?
...then some cheap unscented cat litter which is just clay and then cap this with the black diamond.
Just some options.
We're hoping for solid black if we can find it. Then grey stones (looking to use Feather Rock to get some height (insert joke here about floating rocks), and then let all the colors be plants and fish.
PS. How important is the lack of CEC in the Black Diamond, would a bag or three of Ecco-Complete or something with high CEC do much to balance out under-substrate and in-water fertilizer. I'm going for long term stable, few water changes, low dosing and low tech in the long run.