Hydroballs I found difficult to judge personally, try to get a litre figure for the amount you need and then the amount in a bag, although if you have too much you can use it in other jobs
I don't know what you are making the tank for, but there is a mixture called ABG mixture. It was developed by the Atlanta Botanical Gardens hence the name. It is a very rich mix and some people say it makes plants grow too quick. The recipe I followed was this
2 parts FINE orchid bark
2 parts tree fern fibre
2 parts organic lumpwood charcoal that is crushed (i ran it over in my car a few times lol)
1 part milled sphanum (the stuff in the compressed brick that is a powder
1 part coco eco earth
Mix it all together and jobs a gooden. If you Google abg recipe you will get a lot of results.
Remember to seperate the hydroballs with mesh such as exo terra hydrodrain. Remember to seed it with micro fauna and it should last a good while.
Edit: I told you some people say it is too rich lol