You have made some really good choices!
The filter is a really good one especially for sand substrate.
I like the design that the pump is after the filter media so if by chance any sand is sucked up
into the filter it won't bugger the motor or impeller.
Also the adjustable flow. you want to provide enough flow for proper filtration
but at the same time you don't want to cause a hazard for your inhabitants of your tank either
by the intake or turbulent water conditions that the poor things won't be able to rest in one spot.
I have book marked that model for future use. thanks!
I would keep the sand for a while until you move to a larger tank. ( when they out grow the smaller tank)
While they are small sand will be ok since if they swallow some that can pass it.
Then I would get large gravel for substrate. with a submergible feeding plate like a small terra-cotta dish.
just place their food on the dish.
if you just put it in the tank it might fall in between the gravel and be kinda hard to clean up.
water changes are huge topic of debate among aquarist. it all depends on a lot of factors.
#1 the current water quality of the tank.
#2 the size of the tank
#3 how many inhabitants of the tank and what type (some make more waists than others like ACF'S do.)
#4 does the tank have any live plants. ( I would get some if the tank was larger. that is just me . I always keep live plants.)
I would get a good quality water test kit. like this one,
I would also get a air pump and airstone to keep up the oxygen level in the water.
I know they aren't fish but they do absorb a lot of oxygen via their skin so they don't have to surface so often.
and when they sleep.
Follow the instructions I left on a fish forum on how to set up a air pump and air stone properly.
http://www.aquariumforum.com/134708-post4.html
I would only do 25% water change once a week unless you have water quality problems.
Changing water too much or too often can stress the frogs.
Feeding is also hugely debated issue.
I have fed mine frozen blood worms , guppies (ones I bred my self.) and as a once and while treat.
cut earth worms from a source I trust.
reptomin pellets as a staple.
I fed them every other day when they got to adult size
and when they were small (about the size of a quarter) I fed them daily staple and every other day a few guppie fry
I kept them for 4 years in my office at USC-Aiken so they would go the weekend with out food with no problems.
So did my fish.
Hope this helps