ADF Death!

[VEXEL]

One of our African Dwarf Frogs "Q" Died sometime this week, not sure exactly when. Sometime after Monday night was the last time anyone saw all 5 frogs together. He got caught inside a cavity i didn't know was in one of the decorations and i'm assuming he drowned. Or since I introduced the filter in the tank, maybe it made him a little skittish and scared him. I had been getting concerned i hadn't been able to account for all 5 frogs the last couple days and decided this morning to go on a search. And recovered his body from inside the decoration. I have checked the ammonia and PH levels. Ammonia is at 0.25ppm and PH is at 7.6 temperature is between 70 and 74 degrees Fahrenheit those are the average numbers for the tank over the past couple weeks. (Since i started keeping track) Anyway......is there anything that I need to do to the tank in the way of a water change/cleaning? and what kind of materials are safe to stuff inside this cavity to prevent my other Frogs from getting caught inside this decoration? (and yes i'm keeping this particular decoration out of the tank til i get the problem remedied.)