should i still do a 20% water change after avaporation

How much are you letting evaporate? I do top-offs in between
my water changes.

I would because the waste is still in the tank

Cleaning up the tank is very important.
So i would clean the tank normaly and do the
water change.
How mutch water is evaporateing by week?
It isn't to normal to be losing that mutch water.
This can be caused by the plates in back were you cut the whole to
put the filter in?
Well the bigger the hole the more humidity you
are getting in your house.

Just wondering how many gallons is your tank ?

I would stick to my normal tank and gravel cleaning routine (if this has worked in the past) and just add make -up water as required.

Replacing evaporation doesn't count for water changes since none of the wastes are being removed - just concentrated into a smaller amount of water.

Hi! I do a 5gal water change everyweek in my 30g, which is home to 6 goldfish. I haven't lost a fish to desease in 18 months. I use a 5 gal bucket and file it after every change. I use a syphon to dreg the gravel as I change the water. Most weeks the water being pulled out is very brown with dirt. I don't use an undergravel filtration system so it's important to dreg the bottom for waste.
Good luck and get a bigger tank asap!