Hardy Fish

[haleem8777]

Recently i have been having issues with fish dying, so i was wondering what are the hardiest fish i could keep in a 20 gallon.

 

[fluttermoth]

What fish have you had? Is your water hard or soft? Have you been testing for ammonia and nitrite?

Most fish are hardy; the fact that you've been losing a lot would indicate there's something else going on.

 

[haleem8777]

My nitrates , ammonia is fine, the only thing that changes is the ph recently it has risen as the tap water ph has risen and there's not alot i could do, so i was after fish that would not die everytime the ph changes by 0.5. i got rid of the tetras i just had

 

[SarahR]

Pretty tank.  
As the other poster observed, most fish are hardy.  I have well water and PH fluctuates quite a bit, especially with the seasons (higher in summer/lower winter) but none of my fish are negatively affected.  Not even fry.  You seem to be saying the PH of your tap changes but if you do regular water changes I'm unclear how that would impact your fish.  Without water changes the PH of a tank can definitely become drastically different than the source water, resulting in stress when a change is performed.
If you are testing the source water straight from the tap, you can't rely on those results.  Water needs to be drawn and tested after sitting, preferably with light aeration for a day or so to get a true reading.  I also suspect something else is going on causing your fish losses.
 In any case, if PH shock is truly the reason for your losses there really aren't any species I can think of that easily tolerate rapid PH swings.  You will need to mitigate any swings with small frequent water changes to keep the tank water parameters close to those of your source water.  If you do that, most any species appropriate for the tank should do fine.