Maintaining the tanks of red-eared sliders and other turtles can be a challenge. Here are some ideas to manage water quality, including test kits for pH.
Setup for New Turtle or Tortoise
3 Ways to Clean a Turtle Tank - wikiHow
How to Care for Aquatic Turtles (Such as Red-Eared Sliders) - Petful
How should I care for my turtles? – RSPCA Knowledgebase
How to Take Care of an Aquatic Turtle: Habitat & Food
Turtle Filtration - General Discussion - C.A.R.E.
Here's What You Need to Know About Filtration for Pet Turtles
Is tap water safe for turtles? - Quora
How Often Do You Have to Clean a Turtle Tank? - TurtleHolic
How to Properly House and Care for Pet Box Turtles - PetHelpful
All About Keeping Pet Aquatic Turtles
Turtle tanks
General Care of Aquatic Turtles - CHICAGO EXOTICS ANIMAL HOSPITAL