Just a stab in the dark (Noodle will probably be able to tell you more) but your water company may be drawing from another source for some reason. Water shortage maybe, or they might just draw from different places in different parts of the year?
Don't worry about getting 'flamed'....we aren't that horrible!!
Goldfish are best kept as a species tank. But saying that another member here (jaspersdad) keeps weather loaches with his but he does have a very large tank and big filtration (which is very much needed!)