I have had both salt and fresh water aquariums in the past. I down graded and have a 45 gal
tank with a few goldfish the past 10 yrs or so. Goldfish are hardy and can live in any water condition or so I thought. About 6 months ago the 3 goldfish and a Pleco started to look sick. Eventually all had died. Old age was the reason I believed why. I then purchased 3 Koi but 2 died after just 2 weeks. My wife finally convinced me to test the water. Years of poor maintenance took the toll. Nitrates were +200, nitrites 1.0, hardness 300, Alkalinity 40, and PH 7.2 So here I am relearning and searching for answers. After several water changes I'm happy to report that Nitrates are 80, Nitrites 0, Hardness 75, Alk
120, and PH 7.8. I did add another Koi ( I think its a goldfish as it has no feelers) and a Pleco. The Pleco didn't survive the 64 F water temp but also learned that they aren't good to have with Goldfish. Onward to seek more knowledge...