Can a landlord make up new "rules" that aren't in the lease that was signed?

we just moved into a new place and no where in the lease does it say we can not have a pool. we bought a little kiddie pool for our 3 year old to splash around in but our landlord saw it and told us we can not have it here. the insurance company for the place said it was fine so long as we monitored the use of it and it was emptied each time we were done using it but he still says no. the Water bill is included in rent so i ume that's why he doesn't want us to use it... but if we do continue to from time to time and he catches us can he evict us if it doesn't say no pools in the lease???