I have a few questions and thought some of you might know what is best for me to do.
I have been looking at plants for my 80L tank. It has a 18w T8 light and is temperate at 18-20 degrees. The substrate is pea gravel. The only fish are white cloud mountain minnows.
Should I add a soil substrate if I want plants to grow? How should I go around doing that? Would I need to soak it for a few weeks then add it?

Here are a few plants I am considering:
Hornwort (Ceratophyllum demersum)
Water Milfoil (Myriophyllum spp.)
Water Wisteria (Hygrophila difformis)
Ambulia (Limnophila sessiliflora)
Water Sprite (Ceratopteris thalictroides)
Cabomba Caroliniana
Frogbit (Limnobium spp.) or Salvinia spp.

Also these maybe:
Java fern (Microsorum pteropus) and Java moss (Taxiphyllum barbieri)

Eleocharis spp.
Echinodorus spp.
Apognogeton spp.
Ludwigia spp.
Bolbitis spp.
Blyxa japonica
Rotala spp.

I already have Egeria Densa. So... any I should avoid? I originally wanted Cabomba Caroliniana, but apparently it needs high light?

Hi

Some of those need more light than you have, some need CO2 and some won't like the temperature. Others are OK.

Have a look here where you can paste in the scientific names and find out more about the plants you've listed and you can search using your requirements to produce a list of suitable plants.

Thanks FL! This is exactly what I was looking for and I can't believe I didn't stumble on it earlier as I have been looking through tons of websites. I'll have to revise the list then. Tempted to get a bit of CO2 injection but feel it would be wasted on a low light setup. I wish there was a way of getting more light in :(

Also wish I had thought about going El natural from the start and adding in a soil substrate :(

Hornwort (Ceratophyllum demersum)
Water Milfoil (Myriophyllum spp.) probably Parrot feather (Myriophyllum aquaticum)
Ambulia (Limnophila sessiliflora)
Frogbit (Limnobium spp.) or Salvinia spp.
Water Wisteria (Hygrophila difformis)
Water Sprite (Ceratopteris thalictroides)

My head is mush as I read so much conflicting information, so I'm going to go for the ones in bold. I might try the others on this list if everything goes well.
Any of these sound like a bad idea?
Couldn't find a floating plant that needed low light but since they will be near the light I thought it might be ok? Plus WCMM like floating plants os I thought it would be worth a try.
If I can't get any to work I'll just get duckweed - seems to be a absolute nightmare in many ways, but if the fish don't mind it I won't either!

Hey just a thought! I have salvinia minima and natans and mini water lettuce that will grow in most conditions..i can send you some for the cost of postage if you like?

Amazon frogbit doesn't seem to mind what the temperature is, but it doesn't like being nibbled from below. Salvinia should be perfectly OK too. I wouldn't get duckweed other than the Ivy Leaved Duckweed as its too rampant (Ivy-Leaved duckweed doesn't float on the surface but just below). Water sprite can be grown either as a floater or rooted, and is found in the some of the sme localities as WCMMs are, so it would be biotope correct if that matters to you.

Thanks! Do you think the frogbit would manage if the WCMM nibble on it?
I would love to go biotope correct, but my 18w lighting might not be enough for water sprite :/ but there should be enough light for the limnophilia.

I'm going to start with horwort, frogbit and salvinia then add others in if all seems to go well!

A bit of nibbling won't be a problem. It was getting lawnmowered from below by the American Flag Fish that did for it I think.

18W in a 60-100L should be fine for Water Sprite. Mine grew fine in a 60L with 11W. If it decides it wants to float (and it probably will try and send off floating bits) it'll be fine.

Thanks Dave, I found so much conflicting information I was so unsure about everything. Do you think the Water wisteria would be ok?

Possibly not- the caresheets I've seen indicate it likes thing a bit on the warmer side. I've always gone with the "chuck it in and see" school of planting, which has generally worked out for me. There's only one or two plants I'd really like to grow that I've never had much luck with.