Have a plant which i keep in water in a vase and change the water every
week. Its been happy like that for a long time. The roots are like onion
roots thick white strands.

Recently the leaves are becoming a bit pale, so I guess its short of a
nutrient or two.

A long time ago i heard that putting a copper coin in the water helps to
provide nutrients. True or not i don't know.

Are there any other simple things you know of, that i could put in the water
that would provide nutrients and still allow to me to change the water
easily each week? Thanks.