Plants In Pots

I have been given (through the generosity of Naughty Moose -many thanks - especially as you have a bigger tank to set up) some Java fern on rock + moss and  a couple of other bits, as well as lots of advice.

They aren't a problem as I know they just need attaching which has been done where necessary.

# 2 Grandson now tells me he has some cuttings he is saving for me, don't know what yet but whatever it is  it's hardy.

My last lot of shop bought plants died and I assume it's because I have only course(ish) gravel as the substrate.
As it's only a small tank at 22 litres, I don't want to go to the expense and hassle of taking the gravel out, especially as it will risk my losing some of the bacteria on the stones which I am currently cycling.
I also don't want to add to the substrate and take up some of my water/spacial volume

I did my usual of scouring the web and I'm sure I've seen that it is possible to grow plants in small pots and I've also looked on the plant section here and I know that some people have used soil as their substrate

When I collected my freebie stuff from a neighbour recently, I was also given an unopened bag of RIGA brand Sphagnum Moss Peat (before anyone jumps on me I didn't buy peat - it was given to me).
It says it's a perfect  planting medium and helps to minimise bacteria in a tropical tank so obviously designed for tanks.
I only want a couple of little pots of plants so don't want to go to the expense of buying large bags of stuff if this is usable.  I'm talking maximum of a couple of 2" plastic pots or equivalent. (maybe a shallow food tray with a couple of the plants in rather than in pots)
Soooo......
Could I just use this stuff.
In such a small amount it surely won't affect my PH too much? (PH currently between 7.2 - 7.6)
I would put some of the gravel on the top to hold it down.

Is this feasible please

Two inch pots are very small, I doubt if they would give enough space for the plant roots. I reckon one bigger pot would be better. Sphagnum peat contains no nutrients and isn't a particularly good substrate, it would be better mixed with soil, 50:50 and covered over with a layer of gravel.

As another fish has bitten the dust but ammonia 0/nitrite 0/nitrate 25/temp are fine I'm looking for other reasons I something that I've done and can therefore rectify.
The only thing I can think of is the substrate under the little food tray garden.
I used the peat 50/50 with good sieved potting compost (I went and left the organic soil behind at my daughters). IT is topped with two lots of gravel and no compost came out into the tank.
As it was acceptable to use soil I assumed, probably wrongly, that the small amount of compost wouldn't be harmful.
I see from Naughtymoose's post that his pond soil has been cycling for 2 months and is no longer giving off ammonia.
As my ammonia level is 0 it surely can't be the compost "off gassing" can it?
Should I not have put it in straight away before the fish.
Today has been very distressing. To lose yet another fish since Friday is very sad
What should I do please?

Thanks Richard.
Another dead one this morning.
We got up at 3am because I suddenly thought that maybe a carbon cartridge would help.

I am going to remove the little garden, remove the compost mix and put the plants back in with some finer gravel than is there at the moment.

The only other thing are two stones I put in with the moss on and I anchored them with loom bands as I didn't have any elastic bands. The stones were from the garden, like largish pebbles, but they were boiled 3 times before I used them.
I'm floundering in the dark here.
MA have replied and say they too are completely baffled
Ah well, all a learning experience I suppose