Reverse Osmosis (RO) Units are very useful pieces of equipment for hobbyists looking for complete control over the water they put into their tanks. Tap water contains lots of impurities and whilst this isn't necessarily a bad thing, it can cause problems for delicate fish like discus and marine tanks.
RO Units are simple to use and strip out impurities by water being passed through at least 2 filters. These filters remove the likes of nitrates, phosphates etc leaving your water in a pure form. This pure water then needs to be 'built up' and by that we means minerals need to be added back to the water to give it some stability. However you can use pure RO water for topping up tanks.
The new TMC RO units purify water at different rates - the smaller one's allow for 190L of water per 24 hours, and the largest up to 380L. The advanced systems also contain a TDS meter which monitors performance. You can view the range here.