If you have had your tanks set up since the 8th then you should be getting a reading on ammonia, nitrite or Nitrate.
I would get API Master Test Kit. (It is on sale at petsmart right now).
Also, what tank is that which does not have substrate in it (gravel)?
I recommend getting something other then mondo grass (you said it is an onion plant but I swear that looks just like mondo grass). it doesn't do that great of a job soaking up ammonia. Get Java Fern, Java Moss and Hornwort. You will be happy you did and so will your fish.
Wisteria is good. That will help you out a lot. I have 3 bunches in my 10 gallon. :)
Try and find Prime Water conditioner. That is ALL you have to add to your water when you change the water. All that other stuff is not necessary and they are just trying to rob you. SEACHEM PRIME. The best.
As for the fish from the LFS I would first settle your tanks...see if you can get your money back from them because your fish died.
Once everything settles I would just stalk the first store for a week and wait to see some nice beautiful healthy fish and then get one. This way you are sure your tanks are healthy. You know what you are getting so it wont conflict with anything already in the tank and it also allows you some buffer time to get things settled.
I know you want to rush it but it is easier and better to take it slow. You already have fish in your tank so you are already doing the fish in cycle. There is no point in starting over, it will just be more trouble.
I would just get the three plants I suggested. Hornwort is a life savior. Put them in each of the tank (do not bury the java fern, tie it to something. the roots should be exposed) None of those plants you bury. you can tie them or let me float.
Daily water changes of at least 50% are NECESSARY for your fishies health. If you want them to not be damdaged or die I would do that until you get 0 ammonia, 0 nitrite. Nitrate is safe at a level of 20ppm MAX. anything above that can be dangerous. Don't freak out if you see Nitrate. NITRITE AND AMMONIA should always be at 0.
Good luck and don't worry...fish stores screw us all over...you will settle things down and be happy with the outcome soon enough. Everyone here surely helped me and I am now so happy with my tanks I can't wait to get home every day to look at them :)