Most fish will tollerate a lower S.G. such as 1.017 or 18. Many inverts will not tollerate it though. A lower S.G. is a good way to help reduce parasite out brakes and is a common practice. I don't know anything about your set up (filtration, etc.) but assuming it is set up properly, you can add a damsel at a rate of about 1 fish per about 15 to 20 gallons to start the cycling process. You do not need to wait an extended period of time before adding your starter fish. The cycling process will not start untill you have fish in the water creating pollution. A good practice is after fish have been in your aquarium for a week or two and you have ammonia present, add some gravel or sand from a freinds or local aquarium shops healthy aquarium. This will "kick Start" your aquariums development of benificial bacteria. The reason you wait untill there is ammonia present is because in a brand new set up, there is not (or at least should not) be any waste to brake down and the bacteria culture would suffer. I question why your aquarium has an ammonia reading right now, perhaps double check that. I guess the question I should have asked first is, what type of filtration system are you using and do you understand the nitrogen cycle and how to manage it? I'm happy to help if you got questions, let me know....Joel