colorimeters or spectrophotometers are the only test "kits" I like to use, no guessing, very accurate! A little pricey,but...
The Chemtester CM-750 is a colorimeter capable of measuring water quality using the Aquarium Systems Test Kit Refill Reagents.

Tests:
Ammonia
Nitrite
Nitrate (High Range)
Nitrate (Low Range)
Phosphate
Copper
Marine pH

•The LCD display shows readings in mg/l (ppm).
•User friendly - Self timed menu driven tests.
•Simply follow the instructions on the screen.
at only $329 at J&L its a steal in my books, works pretty good to.

I like to use the lamotte colorimeter and spectrophotometer, but they are $1000 and $2000 respectivley, colorimeter has 40 programed tests with 25 extra programable tests, spectrophotometer does 250 tests and can data log for computer download.

for a hobbiests the chemtester at J&L is a great buy! and then a few salifert test kits to supplement it for reefing