False water test positives?

[FishyFish13]

So I have a 3 almost 4 week old 29g tank with 3 fish. I just did a water test and it came as 0ppm Ammonia, 0ppm Nitrite, 40ppm nitrate. A few days ago it was .50ppm Ammonia and 0ppm Nitrite. Is my tank finally cycled? Yesterday I put seachem prime to protect my fish from the ammonia. Is that why it says 0 for the test? What does seachem really do? PS- I think the Nitrate came from my tap water because I tested it and it did came back as 40 Nitrate.

 

[Heron]

It sounds like your tank is cycled. The nitrates are quite high ( normal after cycling) so I would do water changes to get it down to 10 or less ( some fish are more sensitive to nitrates than others but 10 or less should be good for any)
Prime does two things, it is a water conditioner , this means it removes toxic chemicals such as chlorine, chloramine and metals that are in tap water. Secondly it binds ammonia into ammonium which is less toxic to the fish. Most test kits measure both ammonia and ammonium so a reading of 0 probably means the filter bacteria are keeping on top of the ammonia your fish are producing.
Keep an eye on your tests regularly until the tank settles in. Sounds like your doing the right things

 

[FishyFish13]

Would it be bad idea if I added a 4th fish? My tank stock is a Molly, Platy, Danio. I was thinking of adding a Honey Gourami to the mix

 

[Heron]

Your tank should be able to cope with the gourami. Mollies and platies should be in schools but can cope on their own and danios should be kept in numbers and don't like being alone. You tank is big enough for more fish but you may want to consider more danios