That's your problem there, is only letting it run. That doesn't cycle a tank regardless of what a pet store tells you. From now on... Don't rely on the fish store's advice. Here's what you need to know on how to properly cycle your tank. Since you have fish, your stuck with a fish in cycle. Invest in a liquid test kit, API Master test kit is highly recommended here they can be found on amazon for around $20, test your water and do a water change anytime ammonia or nitrite gets at or above .25ppm. Test everyday atleast (test daily if you want, there's absolutely no harm in testing to much!) or daily if you want. How you know your cycle is moving along... Ammonia will show up first, then nitrite (or sometimes nitrates can show up first with fish in cycles so don't be alarmed if this happens) will show up and ammonia spike then 0 out, then nitrite will rise and then nitrate will show up and nitrite will drop and eventually 0 out (if nitrate shows up first instead of nitrite then is just opposite, in any cycle ammonia shows first, it's just whether the nitrite or nitrate shows up first that varies). Once your readings are 0 ammonia and nitrite and you only have nitrates left your good to go! From this point on you should only have to test once a week and keep nitrates at 20 and below. Hope this helps you!
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