They are usually sold for ponds, not tanks. They're easy to come by as they proliferate quickly.
Oh, I forgot to add water lettuce should be kept in tanks that do not have glass covers or hood. They won't like the condensation drops forming on their leaves. In my experience, they tend to wither when placed in glass covered tanks.
Oh, I forgot to add water lettuce should be kept in tanks that do not have glass covers or hood. They won't like the condensation drops forming on their leaves. In my experience, they tend to wither when placed in glass covered tanks.