We'll u can buy one that i fake but it works with real water it is $100.You can get it at Pier 1

how big? There are plenty of ways to make a cheap waterfall. We used a in the tank filter/pump to supply the water, it has a low pressure high volume pump thats really quiet. then we used aquarium fixtures such as bells and slate to make the falls.

Step 1: Open your faucet

Step 2: there is your cheap indoor waterfall

A blow up pool with a slippy slide is FUN!
Outdoor Fish Pond Pump helps move water, flex pipe with PVC fittings to channel it or move it.
I would put a slippy slide inside.
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