ok so firstly you should read this about
cycling you really need to know about it, it explains what your water parameters should be and how they get to that.
If you've been doing your maintenance correctly then the tank should be cycled after 5 months, however a number of things can cause it to cycle again, such as cleaning your filter under tap water.
when you say you do water changes, do you siphon the bottom of the tank too? how and when do you clean the filter and replace filter media?
the 18" can be made up pretty much however you like.... 18x1" fish, 9x2" fish, 6x3" fish, or any combination to add up to 18. this needs to be worked out on adult size.
to say you can have 25 fish is ridiculous as they're not all the same size, don't produce the same amount of waste or have the same requirements. Yes you could maybe have 25 v small fish at 1/2" each, but obviously not 25 fish getting to 4" each.
there are a lot of factors that affect stocking, however the basic guideline of 1" per us gallon of water works well as a basis to work around.
I think that's too many fish for the tank.
Read the topic I've linked to above, get your water tested and post back with the results we can see if there's any problems. Don't just say the water's fine, get actual numbers for pH, ammonia, nitrite and nitrate.