Hi BDS,
Welcome!
Your experience seems to be fairly typical of what happens – often, generic pet shops have not heard of the fishless cycle or tend to say that leaving the tank alone for a few days is the fishless cycle, while aquatic shops tend to be more aware of what it involves but often dissuade from it for what sounds fairly plausible reasons, especially to someone fairly inexperienced. In terms of sound advice (or lack of it), what is of particular concern is that they sold all 18 of those fish at once in conjunction with knowing that you’d be doing a fish-in cycle albeit with the aid of that product – if they’d sold one of those shoals (probably not the neons which are best suited to more mature aquariums), then that would have been more sensible.
However, well done for keeping up the regular water testing and water changes which are and will be crucial to try to keep ammonia and nitrite at 0, and nitrates low (ideally under 20 if at all possible). The evidence for products such as ATM Colony, Tetra Safestart, Fluval Cycle, etc, is quite variable, so keep bearing that in mind and keep up your regime of water testing and water changes over the next month to six weeks, just to ensure that no “spike” occurs when you might be least expecting it or just when you think that all is safe – there is a greater degree of risk of exposure to ammonia/nitrites and high nitrates with fish-in cycling than with fish-less cycling, hence the need for scrupulous monitoring, and especially with that number of fish.
A few specific suggestions/queries:
• I’d recommend doing the testing at least twice a day eg first thing in the morning and last thing in the evening and, if at all feasible, during the daytime as well.
• What size of daily water changes are you doing?
• What water dechlorinator are you using? Seachem Prime has the advantage of detoxifying the effects of any ammonia/nitrates/nitrites on the fish for 24hrs, and it might be an idea to use that for the next while at least until you’ve got through the cycling process, after which you could always return to using whatever other product you may have bought.
• Underfeeding (eg half a flake per fish per day) during this time, so 9 flakes per tank, would also be wise, in order to minimise the amount of ammonia produced by the fish.
As for the internal filter, what you could do is cut a piece of aquarium sponge to fit in the filter nozzle and keep it in place with one of those twisty wire things (sorry – forgotten what they’re called!) that goes round the top of a loaf of bread. Alternatively, a spray bar can help.
Hope this helps.