Plant Deterioration - Diagnosis & Suggestions?

[MOVED FROM "FLOATING PLANTS..." TOPIC TO THIS MORE APPROPRIATE LOCATION  ]

but it didn't last permanently.

Are aquarium plants meant to last permanently? Aside from anubias (the first plant I got, from recollection), the mossballs, and what's left of the java fern, all of my plants have died within weeks or months at best. (Perhaps the root tabs - other thread - will alter that, of course.)

Plants in nature will last 'permanently'. If they didn't we wouldn't have them to put in our aquariums 

I can't claim I can get them all to last though... I personally believe plants are more picky about water parameters than fish or that they release chemicals which make some of them incompatible with each other... it's a good excuse anyway!!!

I.e. have lasting success with anubias, Amazon Sword, hygrophilia, hornwort, crypts, moss balls. Other I've failed with thrive in others tanks... there is something we are not understanding yet about the underwater aquatic world!

The plant experts will correct me if I'm wrong but my understanding is that a tank fully stocked with fish will produce enough nitrogen in the form of ammonia for the plants. Fish food contains potassium and phosphorus, the other two macronutrients, and this will feed the plants either directly from uneaten fish food or the fish excreting the excess in the food just like we do.
Uneaten fish food, fish poo, bits of dead plant etc end up in the substrate as we know from when we clean it during a water change. Bacteria in the substrate break this muck down and produce carbon dioxide, which is why adding CO2 is unnecessary for all but hi-tech tanks. Those people with a lot of plants rooted in gravel rarely clean the gravel because of the roots getting in the way and their tanks come to no harm from this.
[But those with few or no live plants do need to hoover up the muck in the gravel/on the sand  ]

I have even learned recently that you shouldn't clean up the dead spots under decor as that's where some anaerobic bacteria live.

The plant experts will correct me if I'm wrong but my understanding is...

I'm not a plant expert but I've successfully grown a variety now and what you're saying is correct except the tank doesn't need to be fully stocked as a single fish of around 4 cm with food can provide enough for multiple plants.  I've seen tanks that have no filter at all and the plants and a few fish are kept in balance.  Amazing to see but beyond me at the moment.  With my fully stocked tanks and lots of plants I clean the exposed substrate once a week during water changes as the mess still builds up but the eco system keeps the water quality going for that week.  I could probably go for longer on a couple of tanks but why risk it.

CO2 will speed up a plants growth but is not vital.  If you're trying to get a complete aquascape garden in six months then CO2 will do it.  However, if you want to reduce constant plant trimming leave the extra CO2 out.  I've scrapped the CO2 system I was using on the Amazon tank and the plants are doing great.

@fcmf Looks like you have an emersed form so even if the plant looks to have completely died have patience with it and new leaves will grow.  This strain looks to have been produced from the original echinodorus harbich strain so even if it doesn't go red it's still in its genetic makeup to make good use of iron.

@fcmf Yes they can be difficult and frustrating, IMO water and plants are the tricky things in fishkeeping.
I moved the entire contents of my 200 litre tank into a bigger one at the back end of last year, the only thing that changed was part of the substrate [It was gravel and I now have plant root balls in their original gravel with sand filling the gaps and lightly covering the exposed gravel] However it has taken me the best part of 6 months to settle things down again and getting the plants growing or even not dying.
I almost lost all my Water lettuce, I was down to just a dozen sick plants, and remember I was trying to give this away when it was in the old tank.
I had a green hair algae outbreak that covered and nearly killed a Cryptocoryne forest that had spread to half the old tank. The Anubias was covered in black beard algae and the amazon sword leaves were dissolving and had both algae.
All my filters were choked with dissolving plant matter each week and this added to the maintenance work. This was with the same water, plants and idiot running the system.

The fix was patience and changing single things until I got back to a stable condition. The lights were the first, they were initially completely underpowered so I replaced one and then had too much light... I dialled back the photo period. I have stabilised the yeast CO2 with a second culture bottle and alternate biweekly changes (almost suffocated the fish with a CO2 overdose once) so I now have a air stone running when the lights are off as a temporary measure while I make a new CO2 reactor that will allow me to use my yeast CO2 regulator again and can switch the CO2 injection off during the night. This is all aimed at stable CO2 levels.
Finally I started dosing fertiliser twice weekly (half dose) and have gradually found the correct level.

So don't give up, measure the water parameters (KH and pH) and find out what CO2 level your water has - even if you don't want to add CO2 you need to know how much Carbon is naturally in the system. Check Phosphate levels (look longingly at FKS Coatbridge's small selection of plants, smile sweetly and see if they will do the tests for you ) Have a look at the light spec and see if you can work out the Watts/litre (not a very useful number but it will give a clue if its is light that is needed) If you are not adding fertiliser try a small amount or perhaps add root tabs close to the plant.
If you add the results and comment on the effects to this thread over the weeks (it will be weeks I'm sure) you will get lots of other thoughts as well.
Chin-up and keep planting! If you would like some Water lettuce or Amazonian swords I now have loads again

Glad the root tabs were a good investment for you    Now I need to work on getting you to take them out the pots and plant them proper!!!

I did once have a fish get itself stuck in one of the black plastic plant pots overnight now I think about it... it didn't make it till morning...

No seriously... get your tetra back to health first!...