Because your tank is very young and has not gone thru a cycle yet, it can take 6-8 weeks, any fish in your tank are going to be as risk from building Ammonia and NitrIte levels. These can cause death and any dead fish that are not removed and decay on their own, (this happens VERY fast) will also cause an ammonia spike. Without an abundance of live plants the only way thru the 2 stages is daily water changes especially if you have no way of testing your water. And at 3 weeks and a missing Pleco I would definitely say you have had an Ammonia spike. If you have "Prime" de-chlorinator it is excellent at neutralizing the bad while the cycle progresses. It is a bit more expensive but you use a tiny amount! one cap = 50 gallons.
Have you read?: http://www.tropicalfishkeeping.com/f...m-cycle-38617/
This will help you to understand what is going on.
I also want you to look up the 2 Gourami species, Blue Gourami (Trichopodus trichopterus) Profile
, Dwarf Gourami (Trichogaster lalius) Profile
to see what their minimum tank sizes should be. We have Profiles in the bar along the top or just click the link. I think you will find the Blue is going to get too big and really needs more room. If your Pleco was a "common" then he would have grown to poss 24 inches. A Bristlenose Pleco stays smaller but I'm not sure a 10 is big enough for him either. Eventually 2-3 Oto Catfish (Otocinclus macrospilus) Profile
would work better but you will need an established tank and some Algae growth. Don't get a Siamese or Chinese Algae eater, they get aggressive when they grow bigger and are not suitable for a 10 gal either.
Anyway, I'm really glad you posted a thread! There is lots to learn and we have all made all the same mistakes in our fish careers! Just keep learning and try not to make the same mistakes twice. And past any and all questions you may have!! We love to help... and love pics of the tank too!!
When your budget allows for it, pick up an API Freshwater test kit.