White distilled vinegar top and bottom of shell use a rag or a q-tip "not a soak" , do not get in her eyes . Eyes and lid are very sensitive.
First what are your numbers coming out the tap and or well , know what you are starting out with.
RES turtles are very hardy , feel you may be over thinking this.
Those rocks/gravel are they real ? They in only 8weeks will leach out minerals. Bare bottom is best or silca sand. Or know exactly were gravel/rocks came from , not all gravel rock at a pet shop are safe.
Getting a better filter and regular schedule maintenance will help a lot. Your filter does not have enough flow to mix things up better or hold the good stuff. Until you get another filter may have to per-pare water few days before hand. For decades I used a 100 gallon storage tank of water changes without any chemicals. Once you start using chemicals you will need one for up then down. Remember I’m an old timer , chemicals cause wild swings . Also most like to use driftwood or almond leaves to control those number’s slowly , naturally. You may want to wash /soak some of the tannis out. You don’t want wild swings too fast ! Do you drink the tap water or have an RO system if it that bad. You can use some RO water to get them down if that high. Distilled works too. Are you having wild PH swings ? Turtles breath like us so no real concern about a little high numbers unless off the chart. The guppy loss may of been that 20% water change they never seen before? Get the basics under control and in time can tweak things. What are your other numbers running ? Most of my water changes are 5-15% no more. When I do a full break down about every two years I pre-pare the water ahead of time !
Not like you have a shrimp or plant take good read here :
https://fishlab.com/aquarium-kh/PS: Creativity helps with stock tanks on the inside and outside of them.
I'm old but I know what " XXXXX " stand's for , please ! This is an open forum to young and old.