20 gal tank, ph level

Question
I have 1 male platy, 2 female platies, 3 neon tetras.  We had a case of ick
and wiped out all our fish so slowly starting up again and treating the tank
with maracide.  Prior to this we have had the tank set up for oh i would say
7 years and the problem we always have is the ph level always drops.  I test
the water from the tap and it will be 7.5.  I can test the tank 3 days later
and it will be 6.0  we've tried the tablets and the ph level up drops, we will
do this but the next day it will drop again.  Any ideas why are does it really
matter.

Quick other question, male platy sitting at the bottom of the tank, he was
introduced yesterday.

Answer
hello julie!
the reason for your ick coming back might be because some decor or filter system has hidden the infection and should be removed and cleaned while your cure it.
pH is important because some fish tend to like higher or lower levels of pH (more acidic or alkinity) in the water.
pH tends to change during the night to day about .1 to.5 and comes back as day comes around.
once you've changed the pH you will have to keep making sure it stays that way or else it'll drop too suddenly causing stress or even death.
since there is no "normal" area in pH because different fish like different levels theres 3 different readings.
acidic is one alkaline is 2 and neutral is 3.
usually the tap water is neutral and stick with "if it aint broken don't fix it" rule.
the best way to  fix this is complete reset your tank.
remove the fish in a different aquarium and remove 90% to 100% water change cycle and return your fish then just leave the pH where it sits.
or if you want to continue to fix the pH and want more acidic
"low pH" add increase  hydrogen ions.
if you want "higher pH" increase hydroxide ions for more alkinity.
and for the fish on the bottom he may be stressed give him a day to adapt to his new home.