1. That depends on if you want to cycle your aquarium or not. I'm not sure how familiar you are with the nitrogen cycle but essentially you can "grow" beneficial bacteria that convert toxic ammonia (from waste products, uneaten food, etc.) to nitrite. Over time, a second group comes in to convert nitrite to nitrate. Nitrate isn't too toxic, especially in low amounts. It can be absorbed by your live plants and removed through water changes. A cycled tank doesn't need (and shouldn't get) full water changes. Instead, you might change 10% of the water a week by gravel vacuuming. A cycled tank is more stable than one relying on full water changes so it's something to look into
. There's a product called Tetra SafeStart that kick-starts the process by containing live bacteria. Since you already have your frogs and can't do a "fishless cycle", I would use a product like that or seed your tank with bacteria by taking some gravel or filter media from another established aquarium.
Over time, if you're just doing partial water changes, the tank will cycle on its own. You'd want to be testing ammonia/nitrite/nitrate regularly during this time. I wouldn't normally recommend cycling a tank with animals in it but the live plants may actually keep things safe by absorbing some waste products
. Heavily planted aquariums rarely get spikes at all.
I'm not a fan of undergravel filters for planted aquariums (what kind of plants do you have, though?). I like simple hang-on-back models. Some are even adjustable so you could turn down the flow
2. I'm not a fan of feeding beef heart to aquatic frogs. It's a very unnatural food for them. The guppy fry should be a treat IMO and the beef heart shouldn't be fed (dogs love it, though!). I generally feed what they can consume in a few minutes so yes, you could just siphon out uneaten food. But one of my frogs was a pain and needed to be hand-fed so I had to feed him individual worms
Consider adding blackworms, freshwater mysis shrimp, and/or wingless (or flightless) fruit flies to their diet for variety instead of the beef. Blackworms especially tend to be a big hit with amphibians. Mine also enjoyed the occasional pinhead cricket.
3. I wouldn't do full water changes at all, I would cycle the tank and get a good aquarium test kit. If you do decide to do 100% water changes, I would test the water frequently and immediately do changes whenever you get a positive test for ammonia. Besides that, I'd be changing the water completely every other week or so. But look into cycling first
. There's a sticky in this forum that explains the basics.
By the way, welcome to Paw-Talk and congrats on your new frogs! It's always great to meet someone who truly cares about their ADFs
. Many people get them as "decorations"/gifts now.