water lettuce (Pistia stratiotes) preserving and eyewash?

I read somewhere that we can make a tonic or eye wash from Water Lettuce to treat conjunctivitis - a kind of inflammation often caused by allergies.  But I can't find any recipes.  

It's something I get from time to time and by the time I get a doctor's appointment, the symptoms require strong medicine.  It would be worth trying a herbal approach to see if I can nip it in the bud.  But I have questions.

How do I make this eyewash?
Is the eyewash something that will keep long term or does it need to be made on demand?
How do I preserve this plant so I can use it later (it only grows in the summer here)?
What contraindications or cautions should I observe when using this herb?

Are there any other uses, medicinal or otherwise for water lettuce?  

I've never heard of Water Lettuce being used anywhere near the eyes in y country but there is a reference to it in https://en.wikipedia.org/wiki/Pistiathat in Nigeria and Gambia, the leaf is infused in water to create an eyewash to treat allergic conjunctivitis.[46] The eyewash is known to have a cooling and analgesic effect. Therefore, the plant is commonly called 'eye-pity' in Africa.

This is what I do know of the plant and its uses. My family have used in over 6 generations mostly for enriching soil but also some other uses I'll put in below.

Medicinally The plant is used IN MODERATION to treat swellings and urinary tract infections.
The leaves are diuretic, emollient, expectorant, laxative and stomachic
They are used in the treatment of dysuria and stomach problems
They are mixed with rice and coconut milk in the treatment of dysentery; and mixed with rose water and sugar for treating coughs and asthma
The leaves are used in the treatment of gonorrhoea, probably because they act as a diuretic
The leaves are used externally to treat skin diseases, such as boils, piles and syphilitic sores
They are also applied to haemorrhoids
The ash is used to treat ringworm
A decoction is added to bathwater to treat oedema
The roots are used externally to treat burns. They are pounded and applied as a poultice.
Some caution should be exercised in the use of this plant, an overdose may cause acute diarrhoea.
All parts of the plant contain calcium oxalate crystals plus unknown toxins so ingestion, in large quantities, can cause an intense burning and swelling of the lips, tongue, and throat; nausea and vomiting.

It's quite common here in parts of Australia and this is how we use it. Leaves can be parboiled and added to soups but for me there are better weedy alternatives that taste better as Pistia stratiotes is very high in calcium oxalates, so it would be best used as a famine food. The ashes of the burnt plant can be used as a salt condiment substitute.

My grandmother used  the whole plant with soap for taking stains out of clothing, and this would make sense as it is high in potash.

Also if you are wanting to clean greywater it is a fantastic waste water cleaning agent as it takes up nitrogen and phosphorus compounds, and heavy metals as it harbours active microbial organisms.

My grandparents used it in the pond and at the end of their gulley trap (a shallow drainage pit with kitchen waste that included fats from cooking as well as dirty water before letting the water drain through a rock and reed bed where water that finally drained through to the other end came through potable and cleaned fit for drinking. They had it tested several times over the years after extreme weather events (fire/ash fallout etc which affected the water and land), and each time it came through clean. We used the water lettuce to clean water and fungi to clean the ash residue from our soils after toxic fires.

This has been a really interesting topic to be part of as it has brought back many memories of my amazing grandparents for me.

Hi...Not familiar with water lettuce, but growing up we always used original Lipton tea for conjunctivitis. Perhaps any black tea would work or if I remember correctly, it was orange pekoe tea (per Google Lipton tea ingredient-orange pekoe and pekoe cut black teas).  Steep tea bag in boiling water in cup/mug, let it cool, then use dropper or eye wash cup to bathe or rinse eye several times until film over eye is cleared, several times a day as needed.  Do this daily until eye is free from discharge, redness and itching.  If using eye cup, only use for one eye at a time and discard tea after each use, then wash cup. Do not use same used eye cup solution for both eyes.  Can cover fresh tea batch and keep at room temperature for the day but make a new batch daily. I would use it whenever it would feel like my eye would start itching again or at least 3 times per day.  I never used antibiotics for eyes for myself or my children for any type of conjunctivitis.  It was very effective for us and cleared up quickly.  We also took the standard protection measures of not using the same towels once used and did not blot eyes, when helping to dry, in the same location of the towel, as well as frequent hand washing especially after touching eye.  Pillow cases were changed frequently as well.

I can't help with the water lettuce, but I've always used eyebright (an herb: Euphrasia officinalis), orally, either in capsules or less often, as a tea. I've also used it, in bad cases, both orally and as a rinse. The eye rinse is in the form of a weak tea, made with distilled water.

Used at the first sign of discomfort, it's most often eliminated it, before bed. If used early the first day of visible infection, it has still always worked for my family, within 24hrs. My kids used to get it FREQUENTLY,  from classmates.