A couple quick comments. If you Swordplants didn't do well, it was probably the ultrasoftwater. A CaCO2 hardness of 75 ppm is pretty soft and the soil sometimes pulls hardwater nutrient out of the water. Thus, your Val may have problems as well. I would make sure you add a calcium source to the substrate or add some calcium, magnesium, potassium to the water (my book, p. 87).For plants, from my current 10 gallon, the rooted plants I have are 2 stunted aponogeton and some crypts, a sad hydrocotyle. For stems, I have a ton of rotala. Other water column feeders, 2 massive annubis, subwassertang, guppy grass, some sad java fern, couple mossballs. I have frogbit and duckweed floaters.
Plants I intend to purchase are all rooted plants because those are the ones I don't think I have enough of given the 40 gallon footprint. I want to try valliseneria in the very back (and then plant the rotala stems right in front of it). I'm also going to try an amazon sword or two back there but I've never had success with them...they just melt and don't come back. I want to try a tiger lotus bulb and a red dwarf aquarium lily bulb in part because I can't get a good sense of what the difference is between them online. More wendtii crypts, another hydrocotyle, Monte Carlo and dwarf chain sword to see how they do.
I plan on using a powerhead filter and an air stone and maybe keeping the tank unheated as my house doesn't really get below 66 and I'm thinking of cooler fish. Maybe a heater as just in case for the winter.
For fish, I am thinking of nano fish but in large numbers, clown killifish on top, celestial pearl danios in the middle, cory catfish and pleco on bottom. Maybe a honey gourami once everyone else settles in. Of course snails, ramshorn, trumpets and pond snails because you can't escape them.
I would love any advice or suggestions (easiest to enact at this planning stage!) and will update this thread when things get underway.
Some plants (Crypts, Aponogenton) don't thrive at low temperatures below 70F. Make sure you include some Val spiralis, Bacopa, Anacharis, Pearlgrass, Sagittaria, all temperate plants that do well in cooler water.
Since you're going to use a filter, I'd hold off on the aerator unless you see that your fish actually need it.