i am not sure the fish take strips will work never tried it.. but you can purchase soil testers for pretty cheap... there are also the ones that have the metal prongs at the end with a variety of things, PH soil, moisture, light etc. they run about $30 i think

maybe try flushing a little more thorough meaning with more water, then let dry out for like a week, i am no expert though, just something i would try if i had nothing to lose, cuz if you change the soil you still have all the soil that is tied into the roots that your not changing.