API + tetra test strips are both very inaccurate. I have a liquid test kit and have therefore seen this myself. Also they don't test for ammonia. The best test kit available is from Salifert apparently, (A member on here has tested all the kits against labratory grade tests and has found it to be the best). So seriously...liquid kits ARE easily more accurate, and then they themselves increase in accuracy depending on brand.
I have seen no problems with any guppies put in a pre-cycled tank. So long as it was cycled properly and with a large enough amount of ammonia.
10days is very very rare, Full stop. So you shouldn't have said
So effectively, the OP has done it badly but has accidently cycled his tank anyway.
That was a statement, not a maybe.
I really don't want you to take this the wrong way...but having seen your post beforehand I would suggest that you perhaps spend a bit more time gaining experience. You're on the right sort of lines with 80% of what you say. But little things like forgetting about honey gouramis, saying guppies will perish in new cycled setups...Zebra danios aren't nippy at all if kept correctly (90% of the time)...
And you may have said 'more often than not' but...that's not true. The time it takes to cycle a tank depends very much on to what level of ammonia you are cycling too. On our site it is suggested to cycle 5ppm of ammonia as it has been worked out that this will allow you to add 80-100% of your stocking in one go with no ammonia spike. And if you were to encounter an ammonia spike then there are already huge numbers of bacteria present, so it would only take one or two multiplications to return ammonia, and then nitrite to 0 (normally 12-24hours).
The only time tanks cycle ,'more often than not', in 2 weeks or less, is if they were seeded by a mature filter, or if people are cycling with much much lower concentrations of ammonia. Cycling with lower levels is fine, but then people have to be advised on lower stocking levels. Plus they then need to add new stock in lower amounts over a longer period of time to allow the filter to catch up without exposing the present fish to ammonia.
To Ken Bo:
Seriously, test kit and the add and wait method of ammonia is the best way forward.
You will know exactly to what level your tank is cycled, and for people who have cycled to 5ppm can normally add 80-100% of their fish stock all in one go with no ill affects.
So for example you aim to have 40cm or 16" of fish in a 60l tank (for the first 6months), then you can add that many fish (adult lengths being considered, not the length you buy them at).
An example stocking would be...
2 x Honey Gouramis (6cm each)
5 x Harlequin Rasbora (4cm each)
1 x Pitbull Pleco (6cm )