The temp of 80* is ok especially for babies and adults are hardy and really don't mind. I keep my tank year round 78-80* for my Piggley , keeps him active all the time no slow downs in winter as some may experience.
If you done too large of percentage on the water change it my have affected your "cycle" (killed it off) in the tank causing an bacteria/algae bloom ? Several smaller water changes every few days may help to recover it , that is unless it's a result of the "well" water.
Something I keep on hand is a UV sterilizer. I have a sunsun cup-613 and it only gets used maybe once a year for my 125 gallon tank if that often. Yes it's oversized for what I have but within an hour clears my tank up killing off any bacteria/algae blooms. But handy to have one to me. Also it being matched to its own build in pump the flow rate is balanced to the 13watt lamp , a must for a UV sterilizer to work , proper flow rate over the UV lamp.
How long the well been up and running? When last time your system tested? The smallest change in ground water temp's will change the bacteria level.
Look at the chatrs for sizing one:
http://www.aquaultraviolet.com/sites/de ... 0Water.pdfAlso a good read :
http://www.clean-water-for-laymen.com/b ... water.htmlThat is until you have your well water tested. If you read up on UV sterilizers you will find some that will kill off the blooms / bacteria / parasites all in one. Or you can try several smaller water changes every few days but to me sounds like your well water should be tested if it's been awhile.
A UV sterlizer now will help Krush's tank until you get the well tested. It will kill off the alage... so your filter can remove it easily. I say this because you state even 15' from window and it never happened before. And to me a UV sterilizer is something nice to have for what it can do.
Some people use one when they maintenance there tank's ,just over night, that is if it sized properly to get super crystal clear water. There are other ways for super clear water ,yes, but it helps to take it to the next level. Always a good filter (flow rate) with media matched for a turtle setup helps.