test strips are very inaccurate try and get a liquid based kit like the API master kit. it can be bought from ebay for about £17 and tests ammonia, nitrite, nitrate, high pH, low pH and will last for ages. there are other test kits available but this seems to be a popular one on here.
If you're in the UK get the Salifert test kits, they're the same price as the API's, but are more accurate, they're available from charter aquatics, and you will need ammonia, nitrite and PH.