New to cichlids and already had trouble

I posted this question on the African page but just realized as I was browsing around that this might have been a better place to post it...any opinions?

Hey guys...My husband has been telling me that I needed to find a cool cichlid forum for a few weeks now...so here I am! Around December I decided to convert my long time community tank to Mbuna Africans. I had the help of a LFS as I adjusted my pH up a bit and got the tank ready for them. I started with 9 fish in a 29 gallon tank. All was going very well at this point in time. My husband decided not to use a 60 gal tank that we had purchased to start a salt aquarium, and so it became my new cichlid home. I got them transfered over and happy and healthy for a month or better. Then I added a few new fish from one of the big chain pet stores and started a complete tank collapse. All of the fish in the tank including my 9 originals died except one, plus all the new ones that I added. When they died, they had a slight discoloration to their bodies, starting with the dorsal (I think, the one on the back of the fish) fin. Then they would start sitting on the bottom of the tank and breathing really fast. I was told by the LFS that it sounded like a fungal infection and I started dosing with Melafix. I did a full round of Melafix treatment, yet still the fish died. Since completing the treatment (approximately 3 weeks ago) I have done two 20 gallon water changes. The one remaining cichlid and the catfish seem to be doing fine....

Now the questions... What steps do I need to take to make sure that my tank is healthy enough to begin adding new fish? Should I do some sort of treatment to kill and residual "funk" that is in my tank? The Melafix really didn't seem to do much! It really upsets me to have complete meltdowns in the tank like this, so what ever I need to do now to make sure everything is healthy is much preferable to going through another fish catastrophe!

After posting this, I got one response asking me for more information, so heres my response to that as well!

Ammonia-0-0.25...It might have a little bit of a green tinge to it indicating the 0.25, but I wouldn't say that it is full on 0.25. I hope this makes sense.

Nitrite- 0

Nitrate- 5-10...little darker than 5, but lighter than the 10 color.

pH--7.4

I have started adding "Doc Wellfish's aquarium salt for freshwater fish". Yesterday is the second time that I have added it, and I have yet to add as much as they say to on the box. The directions say 1 rounded tablespoon per 5 gallons. Last water change I added 2 flat tablespoons for the 20 gallons that I had changed, and today I added 2 flat tablespoons for the 20 that I changed as I wasn't sure how much acclimation was needed for my fish.

Temp: 78 deg. F

Not sure what KH or GH are, so you will have to enlighten me.

Using the hydrometer that I have for my salt aquarium, the salinity is just under 6 PPT and the specific gravity is 1.004. When I have calibrated this hydrometer with the local salt store, it shows up slightly higher than what their super special thing reads (.001 higher). So this may be higher than what the water actually is.

I cycled the tank with the filtration from my original 29 (a hang over the back whisper power filter and approximately 1/2 of the gravel from my 29). The 60 was doing great with the 9 original fish for close to two months before I added any additional fish.

The tank currently has a pro clear aquatic wet dry system with bioballs and the blue and white piece of filter foam. It has an overflow box with a white circular sponge at the back of the tank.

For the most part, the fish were eating until they died. Usually they acted fine- swimming perfectly, breathing regularly and eating fine until about 24 hours before they died. At that point in time, they would sit on the bottom of the tank in one of the little hidey holes and would be breathing visibly faster.

They did have white stringy feces which I did notice, but did not figure it was a big problem...I researched it a bit more after my original posting and found that it could be a sign of bloat.

I never seen any "bloating" on my fish...at all! I saw white stringy feces on some of the fish but not all of them. I wish now that I had kept better track of that, but at the time, it didn't hit me as important. I am feeding New Life Spectrum cichlid formula that I was advised on specifically because its good for cichlids and reducing the chance of bloat, but reading nutrition information on the back, I see that it has 34% protein, and a high protein diet is one of the causes....I'm really confused now. I generally fed one small pinch of these small (1mm) sinking pellets once per day when I had all the fish. I just counted the number of pellets in my pinch and it was just under 50, which created a flat (no overlapping pellets) dime sized area on the palm of my hand. So about 3 pellets per fish if the cat didn't get any (which he normally did)! For the cat, I drop about 4 Wardley shrimp pellet formula pellets in every other day. The cichlids would pick at these as they floated to the bottom, but the cat is an absolute vacuum and eats all the pellets within seconds of them hitting the bottom.

Fish that I added and fish I originally had:

Original 9- 3 yellow labs, 3 red top travasa's, 3 socofoli....

Fish that I added- 3 red zebra's- 1 typical, 2 albino; 1 ice blue zebra, 3 kennyi- 2 fem's 1 male.

I realize that this is an incredibly long post, most of which was copied from my other "discussion" on the African forum, but I figured I would get as many details as I had out there from the beginning...to hopefully have a better chance of making my tank "right" again.

Thanks so much!--Especially those who have gotten to the bottom!
~Melissa