My 180L

I also use Profito and it is pretty economical, especially as my local Squires garden centre does a 3 for 2 deal on all EasyLife products. I've also read lots about EI dosing, which uses dry supplements and is even cheaper. Given my tank is only 55l and I use 1ml a day of Profito (less than a bottle a year) I'm not sure it's worth the extra hassle, but if I had a larger tank I would definitely consider the EI method.

It's short for the Estimative Index. A chap called Tom Barr did lots of experiments with plants to see how much of each nutrient they required for optimal growth as he found testing levels in the water to be unreliable. It's explained very well here: http://www.ukaps.org/index.php?page=dosing-with-dry-salts