Wow...got a lot of good advice. It seems too me if your not growing in soil, listen to Bernie 420. If in soil, listen to Lucky Luke.
But either way the main cause of your problem is over watering. The 'finger method' will only tell you if the top inch is dry and get your finger dirty.
Go by weight. Water weights 8.34 lbs. per gallon. My 5 gal. pots ,with soil, can hold 3 gal when dry so when I water I just added about 25 lbs. to that pot.
Easy to check just by lifting the pot, and no dirty finger. Just a wretched back (lift with the legs!).
If it's not root rot then it's a pH lockout, by the way I don't agree to just dumped un-pH'ed water in soil, that's not going to work unless you have tapwater that's 7.0 or so and not many do, my tapwater is 8.9 and if I dumped that in my soil that plant would be dead in a no time!
Like he says, check whats coming out of your tap. Many municipalities are raising tap water ph so they don't leech lead from old water systems. It's a good idea to check by testing or just calling the water dept.