About the septic...
A septic tank would not just empty itself over time. If it was full, or partially full (and totally full is their normal state of functioning) then chances are it is still full 15 years later. If I stopped using our septic system (and believe me, I would LOVE to stop using it and connect to city sewer instead) I would not just leave it as is - I would have it pumped and filled in.
If the lines leading to the system are compromised - cracked, etc. or if the lid to the septic is cracked (it happens, in fact we just had to fix ours) then rainwater could be getting into that tank and making it overflow. And chances are after 15 years your leach lines aren't good, so consequently you're getting sewage sludge bubbling up. Yuck.
Since you obviously want to use that part of your property, I highly recommend - if only for your peace of mind - having a septic maintenance co. come out and locate the tank, pump it, fill it in and seal it off properly.
I also highly recommend ditching the test strips and buying a full test kit with tubes and chemical droppers. They don't cost that much and the results are much more accurate and easier to interpret.
I am a newbie to having my own aboveground pool, but I grew up with one and we had a hot tub for several years that I maintained.