Yes, with 5 bala sharks and 2 oscars in a 10 gallon, the ammonia levels, even if they are very small, will build up extremely quickly, especially in an uncycled aquarium. You may want to browse around this site a bit and do some research on the "nitrogen cycle," bala sharks, and oscars, and you may have an idea of how badly they ripped you off. Just for a starter hint, those 7 fish would require something along the lines of a 250-300 gallon tank to survive and thrive, and you'd still need one hell of a filtration system and water change regime. I think you need to not only return the fish to the store, but have a word with the manager about the gross neglect the employee who sold you those fish for that setup showed by allowing you to buy them, and moreso for saying that it was ok in your tank. I really want to rant, but I won't... I can at least rest assured that I am one of many on here who would love to chew out petco for things like this. Its part of why I miss working at petsmart. I am a very well informed fish keeper... I could not tell you how much research I've done about just about every fish out there. (don't get me wrong, I do NOT know nearly everything) I loved helping people set up a healthy environment for their fish to thrive in, and I was knowledgeable about darn near every single fish we sold. I moved on for a better paying job, but I left feeling confident that the staff I worked with were all well informed and would do their best to continue providing excellent service and knowledge on to the customers. Sadly, several of them have moved on, and their replacements know nothing about fish. Its just one of the unfortunate realities of retail... not many people going into a retail store such as petco or petsmart have a background in animal care, and it is not a prerequisite to be hired there. The few stores that have knowledgeable staff are fortunate, but eventually those folks will be moving on to a better job. :-( Good luck to you kmk420mike. Do yourself a favor, return the fish, do some research, and set up a happy and healthy community. It will be worth the effort. :-D