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This article will help you find the cause:
https://www.gardeningknowhow.com/plant-problems/environmental/what-causes-brown-edges-on-leaves-of-plant.htm

They appear to be a type of spider mite. They can be many colors, but they all take this appearance. They can be brought in from simply stepping into your yard, on clothing, and just by crawling through gaps.

This can be treated, usually with something like neem oil, but there are other ways to control them as well.

This article will help: https://www.gardeningknowhow.com/plant-problems/pests/insects/spider-mite-treatment.htm

This is, most certainly, a deficiency. What other plants share the water? many larger plants can take nutrients before the lettuce can get to it. If you are concerned about a fungal infection, then I would apply a few teaspoons of peroxide weekly to the water. This will stop any infection

Plant Marigolds near your Lettuce beds!
These links will help you.
https://www.gardeningknowhow.com/edible/vegetables/lettuce/lettuce-pest-control.htm
https://www.gardeningknowhow.com/ornamental/flowers/marigold/using-marigolds-to-keep-bugs-away.htm

Those are brassica (the genus that includes both lettuce and radish) seedlings. When you water, those tiny seeds get washed everywhere. A google image search for "lettuce and radish seedlings" will help you to confirm this.

I would recommend checking with sites such as Ebay, Etsy, or Amazon. They will usually ship to just about anywhere. Alternatively, you can check the local nurseries in your country. They may have what you are looking for.