API Master Kits are considered accurate, probably the best test you can get at a reasonable price.

If the test is in error check the expiration date on the bottle.. make sure you add the water to the line. I would heavily shake both bottles and REALLY REALLY shake the second nitrate test bottle, as it tends to crystallize.

Add 10 drops of bottle 1, swish it a few times so it mixes with the water, add 10 drops of bottle 2 and swish it around some more until it's mixed together with the water. Let it sit about 5 minutes and look.

Water wisteria is very good at sucking up nitrate and is a fast growing plant, so is water sprite and pennywort but they all 'eat' nitrates about the same. If you are getting high nitrates still (and it sounds like you're doing everything right) I would check to see if something is 'dead' in that tank, a fish or something may be rotting in there under an ornament. Maybe you should just do a complete inventory of what's in the tank and look for something that may be suspect. Wanna take a picture of the tank? Any driftwood in there? Perhaps it is rotting?

A 29 gallon tank is quite adequate for a single clawed frog, so we know it's not water volume. The filtration may be suspect but I don't know much about Eclipse filters.. how did your Cichlids die? 8 years is a very mature tank, did you have these problems with your previous fishes? Were there any premature deaths there? I would not panic but something is certainly 'odd' here.