They would not eat all their fry, that is an urban myth, how would a species have survived millions of years if it eats all its offspring. I think your tank is very pretty! Low tech and attractive, i want to do something like this myself soon, My Bare bottom tanks are ugly! Where do you find those tanks without frames?
Honestly, anywhere from 76-82 degrees F is fine temperature, maybe you just need to buy some more guppies. You can do the floating plants thing if you really want fry, or maybe also the females can be Chased by the males so much they don't have babies because they can never relax, so always try to keep a 2:1 Female to male ratio. This may not sound important but you get many more babies if you do and babies are so easy and fun to raise.
how many guppies are in this tank? it looks to be about ten gallon, i would just do 1-2 male and 3-4 females in a tank this size, but of course you could put more.
I have been raising guppies for 5 years, and also my dad raised guppies when i was a child and i watched him, so you can take my advice as sound. Once again PRETTY tank! i love it, was just thinking last night how americans spend too much money on fancy setups for fish when all they really need is plastic tanks and some gravel/airstone! All of your Good bacteria will live happily in the gravel, and its easy and fun to look at not like some kind of hospital room like we raise our guppies in america. lol