Two of my Dwarf Powder Blue Gouramis just died. What happened?

Question
Hello Karen,
   I have a 20 gallon tank, and I've had it for about 6 to 8 weeks. We cycled the tank for a week before adding any fish or plants. We initially picked up 2 Powder Blue Gouramis, and Orange Swordtail, and 3 Molly's. A week or 2 later we added 6 Black Phantom Tetras, (One disappeared, we have no idea to this day where he went) A Black Swordtail and 4 Otto cats.
All our fish appear to be healthy and happy. We feed them a staple flake food, and bloordworms or frozen shrimp as a treat some days. We feed once a day, and make sure were not over feeding, because theres nothing left after 3 minutes. The tank also has live plants.
A couple days ago, I found my male gourami upside down, lying against the castle, I thought he was dead. To make sure I moved the castle away from him and he floated up and starting swimming like normal. However, tonight, I found the female dead, on the outside of the filter, it was trying to suck her in the intake. The I tried to find the male, and he was dead inside the castle.
I do a 20% water change each week and look for diseases. However, I don't do water tests, do you think this could be the problem?

Answer
Hi Sarah,
I am so sorry to hear about your Gouramis. It could possibly be that even though you've had your tank for 6-8 weeks and have been doing a 20% water change each week that your tank could have had a problem with ammonia levels being too high. Even though you waited a week before adding any fish in the beginning, a tank won't completely be cycled until there are fish added to provide the ammonia source for the bacteria to get started. The entire cycling usually takes around 6-10 weeks to complete. It might have just been too  much ammonia from all the fish and there wasn't enough "good" bacteria yet to handle the load. Water testing will help you to be aware of any water conditions that might cause problems for your fish. I would suggest testing for ammonia, nitrite and nitrate levels just to be sure things are healthy. Water should not contain any ammonia or nitrites. Nitrates should only be below 20ppm.
I hope this helps.
Karen~