Don't even try a big plastic container, 2 liter bottle works GREAT! For a 40 gallon you should be able to keep just one 2 liter, but that is dependent on how many plants you have. On my 135 2 -2 liter bottles weren't even close to enough. Pressurized was much easier, and in the long run it will be much cheaper. If you want to get just the reg/solenoid/pin valve combo, then you can build a cheap reactor, and a bubble counter, quite easily. You can get that combo for around $70-80 online. For the fully automatic expect about $160.