I would give anubias a try. Instead of planting it directly into the substrate, tie it to a rock or piece of driftwood. It'll eventually attach on its own. There are a bunch of anubias species and all seem to thrive in low-light, low-nutrient environments. Anubias is almost impossible to kill--we have a freshwater community tank with annoying tetras (red-eyed tetras) who destroy plants and they can't kill them! The plant has tough leaves and supposedly tastes bitter.
You've probably tried them already but if not, try java ferns. They're not edible for most species and don't mind being beaten up or dug up from time to time. They can actually survive when just floating around but I'd tie some to a decoration. Again, they'll start attaching on their own. Plants that don't live in the substrate are really your best option since the frogs will probably dig up rooted plants.
Water sprite is another possibility. It's easier to destroy than the other plants I mentioned but it does very well floating at the top. Since the frogs probably don't spend as much time at the top of the tank, they may leave it alone. It's also very, very fast-growing (so it can recover easily from occasional damage). It can be planted in the substrate but they'll probably just dig it up.
If you can get a rock or piece of driftwood with moss already attached, that could work. Flame moss is my personal favorite but java moss is great too. Both are hardy and don't need much light (then again, mosses in general fit that description!). Loose moss not yet attached will just get beaten up so it really needs to already be established.
And you can never have too many moss balls!
Those guys are favorites of mine, too!
EDIT: I got so excited about live plants that I forgot you had another part to your post, oops. You could look into canister filters (Eheims being my favorites). They can be annoying to clean IMO but then again, they don't need maintenance as often as HOBs. They provide excellent filtration and can be customized with various types of filter media. It's also nice that the filter itself can be stored under the tank out of sight. If you'd prefer another HOB, I would just get one that's rated for a larger aquarium than what you have. That's the easiest way to upgrade your filtration
. I like AquaClear filters but they're not the same quality-wise as they used to be, unfortunately. If you can get a good working one, they're great, just save the receipt
. I have one that's been used for 6 years and another for 4 years. But I also had one die in six months and another leak after three weeks. Kind of hit or miss (same goes for the Whispers). When it comes to HOB filters, they all basically do the same thing so I don't pay much attention to brand now, I just get an over-powered filter that has room for my own filter media. The only ones I don't like are the Aquaeon filters. We have one on a 10-gallon freshwater set-up and it's weak.