I might fill a bucket with tapwater and test the pH after the water in bucket has set overnight.
This will give you an idea as to what type of water the tank will be Acidic,or Alkaline assuming you will be using dechlorinated tapwater for water changes.
Would look to tetra's if pH was much below 7.4 and hardness GH was much below 10 degree's.
Guppies I have kept did poorly in soft water and performed much better in hard alkaline water.
A quarter cup of crushed coral in the filter (mesh bag) for your size tank, may be the way forward if your heart is set on guppies and water is acidic.
Crushed coral may need to be cleaned or replaced once a month for it may(prolly), become covered with silt,mulm,algae,bacteria, and thus be less effective.
This is only material from filter that I would clean under tapwater or simply replace.