Check out the link in my signature, in this post below the snail pic and "Welcome" for a really useful bunch of information on how to have a cycled tank / aquarium, which is critical for fish or turtles. The nitrification cycle, is what builds a colony of beneficial bacteria which processes the build up of ammonia, which results from the leftover food and fish or turtle waste changing to it's decomposing process.
Think food fish or shrimp being old and smelling like acrid ammonia, then rot.
The ammonia, nitrite and nitrate (which there are tests for in the fish section) will be in unsafe amounts if there are no, or not enough BB/ beneficial bacteria to break them down safely.
That can cause blindness and ammonia burns / open wounds, infections (illness, including death) in turtles, or fish.
Water quality is one of the most important things to know about, and not having good / safe water quality is one of the things which causes so much death, diseases and problems, leading to (especially new) hobbyists stopping the hobby.
Personally, I want you and your step daughter (and family) to have a really awesome aquatic animal keeping experience and love your hobby.
A liquid test kit is very helpful. Test strips are often unreliable for knowing accurately what the water conditions are for a tank of water and whether it is safe for your inhabitants.
Once I lost nearly a whole tank of fish because of a test strip not reading one of the parameters accurately, saying it was normal when in fact the tank over night killed a bunch of fish. I had even compared the test strips a couple weeks before with the liquid test kit which tested perfectly, and matched the liquid test results. So I trusted them as accurate.
Checking also if the turtle is getting correct type of lighting. They need special light spectrum.