I've finally ordered a Nutrafin CO2 kit (well done eBay, first time I've seen something cheaper than the shops!). I've never ever tested CO2 levels with my previous Tetra Optimat kit but am thinking that I should really test once I have a more long lasting/permanent CO2 input. Are the permanent tests a good investment? I will likely be upgrading to 2WPG shortly (instead of 1WPG) so I would like to get a reasonably stable level somehow. Any recommendations for entry-level kits?