Dead frogs, please help!

[Jtun]

I'm new to the aquarium life! In November I got my son an African dwaf frog, twinbar, sunburst and a fancy guppy. The frog died within 2 weeks and the sunburst soon after. I realized my water levels weren't too good and worked for a month on fixing and retaining excellent water. I bought another frog on boxing day and I woke up this morning to him dead thankfully my son didn't see. My levels are good, the other fish seem fine (Although I have noticed the twinbar being very aggressive lately) and I've been feeding an alternation of pellets and blood worms for the frog. I don't know what I'm doing wrong, can someone please help?

 

[Aiken Drum]

Frogs and fish arent typically a good mix.

What are your water parameters precisely? Whats a normal water change schedule?

 

[Jtun]

Frogs and fish arent typically a good mix.

What are your water parameters precisely? Whats a normal water change schedule?

I usually do a 30% weekly water change. My levels are 0 ammonia, nitrate 20, nitrite 0. My ph levels were a bit low around 7- I was told it was due to my frequent water changes earlier on.
When purchasing the frogs I was told they're good with other fish. But then again I went to a generic pet store and maybe they're not as knowledgeable as they should be.