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It is a term frequently used when watering inside, potted plants. water from the top means to water the potting soil so the water percolates all the way down. Water from the bottom means to water by pouring the water into the saucer and letting the plant absorb the water from the bottom up.

Note: when watering by filling the saucer, it may be necessary to do so multiple times. Water again until there is some water unabsorbed in the saucer but stop watering from the bottom after about 30 minutes. Also note that salts can move upwards and accumulate when watering this way but not when you water from above, something that helps leach the salts from the potting soil. Instead of watering from below "always", some people prefer to water mostly from below with a few waterings from above now and then in order to avoid the salt accumulation problem.

It looks more like a nutritional problem. Do you fertilize? They are heavy feeders. Use a fertilizer for citrus trees that is high in nitrogen during the summer months. Here is more:

https://www.gardeningknowhow.com/edible/fruits/lemons/growing-meyer-lemons.htm
https://www.gardeningknowhow.com/edible/fruits/lemons/lemon-tree-leaves-turn-yellow.htm

It depends on your houseplant. Some plants are sensitive to the additives in tap water such as chlorine or fluoride and should be watered with distilled water. Most plants, though, don't mind tap water. Your outside hose water would be the same as the tap water. Rain water, if you catch it, won't have the added chemicals in it like tap water so your plants probably appreciate it.

https://www.gardeningknowhow.com/garden-how-to/watering/using-distilled-water-on-plants.htm

Self-watering pots do not work for every plant. For example, they should not be used for succulents or other plants that need to dry out between waterings. But plants that need evenly moist soil, but not saturated, may benefit from a self-watering container.

The difference between a self-watering pot and a regular pot and saucer with water is that the self-watering pot has a wicking system that regulates the uptake of water. Saucers filled with water can allow the soil to absorb all the water at once, essentially saturating the roots and leading to root rot.

Here is more: https://www.gardeningknowhow.com/special/containers/what-are-self-watering-pots.htm

Most are a synthetic material, now, unfortunately. You will likely have a hard time finding paper tape, these days. We do not recommend specific places for purchase, or specific products. I would recommend asking your local garden shops or greenhouse what drip tapes they have in stock. Likely, it will be an updated material.

https://www.gardeningknowhow.com/garden-how-to/watering

I'm not sure what you mean by "pool stock" but I think that is a question for the city or whoever is providing the water. I wouldn't know what chemicals might be in it, but it is worth investigating.