Substrate: When do you need it?
Before discussing how much substrate a hobbyist needs, let's look at the times when you may not need it, and then we will look at the times when you do. I know of two particular situations where a hobbyist would not want substrate: first, some breeders prefer to have a barebottom tank for fry and second, in some cases a barebottom tank is easier for breeding. I will leave the specifics of these two situations to other articles by experienced breeders.
I suppose the main reason beginning hobbyists use substrate in their tanks is an aesthetic one. I believe most people, myself included, think that a gravel or sand of some kind in the bottom of the tank looks better than nothing. This is probably the most superficial reason to use a substrate, but an entirely valid one as well.
The right substrate will also help to buffer your water and, if chosen correctly, will help the hobbyist maintain his or her desired pH levels. This is especially important for African cichlids and saltwater aquariums.
A substrate is a necessity for almost all planted tanks. I suppose you wouldn't need a substrate for a tank with some driftwood with Java moss and Java fern on it, but who does that? A good amount of substrate is recommended for planted tanks (59 cm) in order for the plants to establish a root system. There are all sorts of ways to accommodate a plant, substrate wise, and again I will leave that discussion to a more experienced plant keeper.
So hopefully you, as a hobbyist, now see the need (or not) of using a substrate in your tank. But how much? Let’s discuss how to get the amount you want without guessing.
How Many Kilograms of Substrate Do You Need?
I hope this helps those of you who want to know how many kilos of a substrate you need to meet your desired depth, no matter what shape or size your tank is.
The first thing you need to know is the area of your tank. The area of a square or rectangular shape is the length multiplied by the width (if your tank is square, the length and width are equal, so any side multiplied by itself will give you the area of your tank).
For example: a standard 60 L tank is 60x30x33 cm, so it has an area of 60x30 = 1800 cm^2 (centimeters squared). Once you know your area, you simply multiply it by your desired height of substrate. So if I wanted 5 cm of substrate in my standard 60 L aquarium, I would need enough substrate to occupy a volume of 5x1800 = 9000 cm^3 (centimeters cubed or cubic centimeters), which is equivalent to 9 liters.
Note: I will give some formulas showing how you can figure out the volume of your triangular, circular, or hexagonal shaped tank later. Again, once you have the area of that tank, you simply multiply it by your desired height of substrate to figure out the volume you need. As for you folks with bowfront tanks, take calculus and figure out your own area... ;)
Now you know your volume, what can you do with it? What you would need to do next is figure out the density (weight to volume ratio) of your substrate, but luckily I've done this for you. :) It should be noted that the size of each individual grain of a substrate will vary per bag, so we are not dealing with an exact science. The following volumes will give you a rough estimate per kg of what I will call fine substrate and medium substrate. We will not be dealing with gravel or any substrate consistently over 5 mm grain size.
Fine Substrate (.25.50 mm): 1 kg occupies about 600 cm^3 = 600 mL

Medium Substrate (13 mm): 1 kg occupies about 1000 cm^3 = 1 L

Some examples of fine substrate: Pool filter sand, Seachem Meridian.
Some examples of med. Substrate: Seachem Flourite, Caribsea EcoComplete, small pea gravel.
So how can you bring it all together? I'll continue with my earlier example of the 60 L tank. Suppose you wanted to have a planted tank, and you decided that 5 cm of substrate was enough, you would need 9 L of substrate, i.e., 15 kg of fine substrate or 9 kg of medium substrate. So the formula is as follows:
(L x W x DH (desired height of substrate))/Volume (choose the fine or medium substrate)
Formulas For Figuring Out the Area of Your Tank:
Squares and Rectangles
These are your basic aquariums; most of you will have one of these.
Square tank: length of any side multiplied by itself (L x L)
Rectangular tank: width multiplied by length (W x L)
Triangles
This gets a little trickier; thankfully, most triangular shaped tanks are right triangles with equal lengths as they are designed to fit into a corner, and that type of right triangle is just a square cut down the middle through two corners, so we can deduce the following: any of the two sides (NOT THE FRONT) of your triangular tank multiplied by itself and divided by 2 will give you your tank’s area:
Triangle tank easy way: (any side except the front) multiplied by itself then divided by 2 or (L x L)/2. (do not apply this method to random triangles, it WON'T work)
Triangle tank normal way: (FRONT length multiplied by the NORMAL WIDTH) divided by 2 or (L x NW)/2. The normal width is the smallest distance between the front glass and the opposite corner of the triangle (and it's perpendicular to the front).
Hexagons
This isn't as tricky as you may think. A random hexagon whose opposite sides are parallel is really two triangles and a rectangle (or square), so all you need to do is divide your hexagon tank into 3 different parts. The area of a rectangle is easy (L x W); now figure out the area of your 2 triangles (1/2(bh)) and add them to the area of your rectangle. If you have a regular hexagonal tank (all sides and angles are equal) then there's a very simple formula given below.
Regular Hexagonal Tank: 2.6 (L x L) (side length)
Random Hexagonal Tank: (L x W of rectangle part) + (2(L x NW)/2 of the triangle part).
Circles
This is for cylindrical tanks (not bowls!). It’s probably difficult for you to figure your radius, so what you need to do is measure the diameter of your tank. Make sure your measuring tape goes through the center of your tank; your radius is half your diameter (R = D/2).
Circular Tank: Pi * R * R (3.1416 * radius * radius) or Pi * D^/2 * D/2.
Again, once you've got the area of your tank, multiply it by the desired height of substrate (DH) and you've got the volume you need. Take the volume and divide it by the volume of 1 kg of the substrate you want (fine or medium) and you'll know how many kilograms of substrate you'll need. One last example:
I have a 200 L tank, and I want to put 8 cm of substrate in it. I decide to use Seachem Flourite for the tank. That product is classified as a medium substrate, so I know it has a rough volume of 1000 cm^3 per kg. The dimensions of my 200 L tank are 100x40 cm). The area of my tank is 4000 cm^2. I multiply this by my DH and I get a volume of 8x4000 = 32000 cm^3. My volume in cm^3 divided by the volume of 1 kg Flourite equals 32 kg (32000/1000) of Flourite, which can be purchased (last I checked) in both 10 and 7.5 kg bags.
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