This is probably a dumb question, but I don't suppose there are any shortcuts to puchasing multiple decorations, are there? I see some people have highly decorated tanks. I tend to keep mine to a minimum (91 - 93% for selling tanks and far less for non-seling) because it's such a pain to buy stuff. To purchase and place one simple clam, for example, you have to tap:
Menu - Store - decorations - tank basics - scroll to clam and tap buy - tap and drag to move, etc.
That's a minimum of 6 taps for each clam. If you don't know what you want, it takes a lot more tapping and scrolling to shop around.