GreenAqua had a calculator, which I cannot find at the moment, but they do list this formula:
"How much General Plant Substrate should I buy?
It is relatively easy to calculate how much General Plant substrate will fit in your aquarium. Multiply the bottom area (width and depth - not height - centimeters) of the aquarium with the substrate height (a minimum of 4-5 centimeters), and divide it by one thousand. The result will show you how many liters of substrate you will need. Here is the formula:
Amount needed in liters = Width (cm) * Depth (cm) * Desired Substrate Height (cm) / 1000 (Excel calculator - download)
Example: a 60 x 35 cm base aquarium will need a minimum of 60*35*4/1000=8.4 liters of substrate, so you will buy one of the 9 liter bags."
This is for a flat substrate, while most scaped tanks bank/slope up the substrate towards the back to some degree.
I would recommend this, it gives you more surface area of the substrate for planting and makes the tank look deeper.
You can fill the bottom back with some cheaper material like pebbles or lava rock and just cover the top of the slope with soil, that will be easier on the budget and seems quite common unless the aquascaper is sponsored (getting the soil for free). Then they seem to pile it in like its sold at the dollar store
Just make sure you account for a deep enough layer of soil, I would go for no less than 5 cm to plant in personally but you can check what others use.
Not sure if youre planning on sand in front but the front of the substrate doesnt need to be very deep unless you are planning to plant heavily there, and then still you might get away with 3 cm or so.
Dont bash soil, I dont think it likes it and it may cause it to break down faster, this is just my theory though. Powder is best for top dressing because of the smaller grain size, it holds down small plants easier. You could do something like 3-4 cm of normal soil and then 1-2 cm of powder soil on top maybe?
Wow thank you so much @Hufsa ! That's absolutely perfect, just what I wanted.
90 x 50 x 8 / 1000 = 36 ... so with 25kg already, I need at least one more bag, with two being ideal (let's be honest, I need more than this and probably 2 bags, any extra can go in the next tank). I do really like the highbank look, so will do what you said and get some lava rock or something to fill up space! I am planning on using nice millenium rocks as well to get some drama with the height, I'm planning no having a very low sand area covering about 1/4 of the bottom (still planted a bit), with the rest all being planted with 1 very steep slow, and one more gradual but still high sloop. Aw man I need all the soil! This is what happens when you want stuff like the professionals who get everything for free!! I suppose they are doing their job well at getting me to buy this soil.
Potentially crazy question... does anyone ever use organic topsoil (as I've used in my 60L) underneath aquasoil? It could go in a net bag so doesn't interact too much but roots could still get in.
Thank you!! I think it might look a little too "natural" right now, but since I'm having to overstock it with fish for the new scape, having it be jungly is very useful to keep them all happy. I do quite like just dropping a few leaves at the surface, before they sink it looks particularly real somehow, the leaves just give it that slightly abandoned natural look.
Of course you want me to get the one that's £20 more per bag! I will trust in your experience with it tho. Is it cos it's harder to plant small plants?