I bought some Seachem Flourish Tabs for my plants. I want to make sure I use them correctly. The box says to place them every six inches. I have a couple Amazon Swords, Moneywort, a few crypts, some anubias - should I just place them between the plants under the sand so that multiple plants will benefit from one tab? Also, the box says that the tabs should be replaced ever 3-4 months - will there actually be a tab to take out/replace or will it dissolve completely?