The ammonia is probably too high.
Perhaps you added too many fish at one time and spiked the ammonia. It also takes about 6 weeks for a tank to cycle so it's going to go through that.
Red gills, red dots etc is all a sign of ammonia burns. You might want to consider getting rid of the...
A 6 foot tank is about 80 gallons?
I wouldn't think so. An 8 foot tank would be preferable, like 125 gallons. They get 12" and very active. Like Silver Dollars they often bash their brains in from running too fast into the glass.
I've owned a couple of Red Tailed Barracuda's. They are solitary animals, getting a foot in length and need at least 75 gallons. They have little minute teeth and are predators. Pretty interesting fish to own.
You might want to consider a dimmer. I know my fish go a bit crazy when I turn on the lights and I think it's just the natural reaction of startling someone. If you were sitting or sleeping in a dark room and someone turns on the lights your reaction would be sit up or get up.
As long as there are no dyes, paints or the like you will be fine. Make sure there are no nooks and crannies that some unfortunate fish can crawl up in and get stuck on.
I had a waterfall decoration in my aquarium and the fish would try to crawl up it and get stuck.
I'm not very knowledgeable on cichlids. It looks really well set up and I love the rocks. Where did you get them all from?
All I know is be very, very careful with that Duboisi and Red Zebra those guys are like club bouncers. My father owns his own 100 gallon cichlid tank that he loves very...
I live in a smaller house now but you're lucky you are house hunting.
My buddies and I are saving up to build a couple of houses of our own on a piece of land here but to do so I'll be living in a tiny, itty bitty trailer on the land...so the fish will be in storage. D:
The tank has not been cycled long enough. Look at the highlight Nitrogen Cycle and click the link. The Nitrogen cycle takes, at the least amount, 6 weeks to complete.
When you have higher bio-load fish like Platies and Mollies it'll jack up the ammonia. They may or not make it because the...
I would slow down on the stocking. Baggai Cardinal's need more space, 30-40 gallons I believe, and prefer groups of 3+.
For a tank of 20 gallons I would just do the clownfish and the watchman goby.
Ryanr or someone with more saltwater experience might have a better opinion on this.
1) I don't think so. Often it can just be in your water depending on where you live. This is why I recommend conditioning with Seachem Prime which will get rid of Ammonia, Nitrites and lower high amounts of Nitrates IF there are any. Test your water with the strips and you will find out.
There aren't many options for that tank size. I would go with 5-6 endlers maybe, cautiously paired with a Betta. My endlers and Betta get along very well but it really depends on their own temperament.
Even the smallest killifish need at least 10-15 gallons of space.