FAO Sue - Quick Cycling Q

Your results today are a bit odd. If the ammonia has dropped, it should have gone somewhere. Your results show 2ppm less than you started with, and that 2ppm should have been converted to around 5ppm nitrite or if the gravel you added also contained a lot of nitrite eaters, you should have around 7ppm nitrate. But your results are zero for both nitrite and nitrate. I know that the nitrate test is only ball park, but if those 2ppm ammonia had been converted to nitrate, you should have a reading somewhere above zero.

The gravel would have some bacteria on it, enough to seed the cycle especially if you put it into the filter. Would it be possible to steal some filter media from the 240l? That would be much better than gravel - and you asked about that a couple of days ago 

But for now, since the ammonia seems to have vanished into thin air (water?), I would add another 2ppm ammonia to get back up to 3ppm (and check 30 mins later) and see what happens after another 3 day gap. That would make 5ppm ammonia added so potentially that would turn into 12.5ppm nitrite which is below the ~15ppm where it stalls the cycle. 12.5ppm is off the top of the scale of all the testers I've heard of. If yours really is that high, you might get an odd colour reaction. The way to check if it is off the top of the scale is to dilute a sample, 1 part tank water to 2 or 3 parts tap. This won't give you a terribly accurate result (you need distilled water to dilute it and very accurate measuring equipment) but it would tell you if the reading really is off the top of the scale with pure tank water.

Did you check the ammonia reading half an hour after you added it to see exactly what it showed?

Gravel does have some bacteria on it, but not nearly as much as filter media (unless you have an undergravel filter, but those are not common nowadays). But unless the gravel, or anything else transfered from a mature tank, is in the filter it doesn't help much with the seeding process. The old media/gravel/whatever needs to be in direct physical contact with the new media for the bacteria to transfer because the bacteria grow embedded in biofilm and can sort of spread into the new media. The bacteria also grow better in filter media as those are specifically designed to make a good home for the bacteria and they are in the place bacteria prefer to grow (good water flow bringing food and oxygen, and in the dark). Old media should be placed in the filter first in the water flow so that any biofilm or even bacteria that become loose are washed into the new media.
Putting gravel on the bottom of the tank will speed up the cycle, but not nearly as much as filter media inside the filter. Gravel can be used in a bag inside the filter but this is only really practical with externals.

Well done! That was quite quick for a fishless cycle so it looks as though the gravel did help - or did you use mature media in the end?