Next important thing is the Nutrient Solution. There are two approaches: either to buy a ready-to-use one, or to make one yourself - and pay 20 times less, not to mention being in full control.
Let's start with a ready to use solution. I have experience with "Flora": Flora Gro + Flora Bloom + Flora Micro, dosages are on stickers, clear and easy). I bought 3 bottles, 1 liter each, which is approximately enough for 6-8 tomato plants, from the seed to the harvest.
Buying them was justified, i had to get some initial experience with hydroponics. Right now I use DIY approach.
DIY approach is way cheaper.
Thte solution should be kept in a dark bottle (I have painted a Coca Cola one) or in a black 10 liters bucket with a lid. The reason is simple: if the light goes in, algae will grow, eating all the nutritious components.
Here is a convenient way of weighting salts: use paper origami:
More on page about Calculators.