Problematic Tank - Fish Dying

[JohnDyer]

I have a 56 gallon freshwater column aquarium. It has been stocked for about 6 months. It's heavily planted, many nooks and caves, and an open space on one side.
Equipment
Marineland 350 Canister Filter (Not turned on fully)
Marineland Stealth Heater (I don't remember the wattage, but my temperature has been stable at 80 degrees F)
I've been doing a 25% water change and carbon change every month.
Inhabitants:
1 Rainbow Shark
1 Chinese Algae Eater
3 Green Tiger Barbs
6 Neon Tetras
1 Blue Dwarf Gourami
4 Male Guppies
1 Female Guppy
1 Angelfish (about 2 inches)
1 Cherry Barb

I know, these fish are probably not compatible, etc. The CAE, Rainbow Shark and Cherry Barb I inherited from my parents, and I'm not willing to just throw them out into the world. The Rainbow Shark generally stayed in his rock all day, and only came outside at night to eat. The tiger barbs chase each other, and scratch each other up, but all 3 have a big appetite, and seem to share the pain equally. The guppies generally swim together (not sure if this is normal) All fish eat well, and are fed several types of flake, pellet, and freeze-dried foods. I try to feed twice a day, but occasionally I can't make it during the morning. The tank is in a well lit and well ventilated area. I use some (old) Tetra Aquasafe. The plants had some leaf rotting at the beginning, but now grows well.
I've had several deaths in since I set up:
1 Blue Dwarf Gourami and 1 Neon Tetra - they were part of the first group of fish. I was a little over eager, and decided to buy fish 2 weeks into cycling, since my water readings looked OK. Completely my fault.
1 Blue Dwarf Gourami - It got constipation, it's belly was all swollen, but the scales didn't pinecone like dropsy. I tried to feed peas, add salt, change water, increase temperature, but it still ended up dying. Anything else I could have done?
1 Male Guppy - This guy was looking off right when I got him, but he was so colorful I couldn't resist. A bad decision.
1 Male Guppy - This guy was a fancy tailed guppy, and it couldn't move quickly for it's life. It was kind of left out by itself, the other guppies wouldn't swim with it, and he just kind of withered away.
1 Angelfish - It got stuck in a decoration. I'd heard it happening, but didn't actually believe any fish would be that dumb.

Most of these I understand, and I know what I did wrong. However, recently I've had a string of deaths, and I'm not quite sure why. I haven't added any new fish. The only change is that the Rainbow Shark and CAE have begun fighting, but as far as I've seen, they only bother each other, as the other fish swim in mid or upper levels of the tank. My Ammonia and Nitrate levels are at 0, and nitrite level stays between 20 and 30 ppm.

First, one of my Neon Tetras died. It had been looking a little off, and separated itself from the school, but it still ate, it didn't have any obvious diseases, it wasn't scratching, it still swam normally, etc, so I wasn't sure what to do. I did a 10% water change.
Then, my angelfish died. It was looking perfectly healthy the night before, and in the morning. When I came back in the afternoon, it was laying at the bottom gasping, and an hour later it was dead. No torn fins, no scratches, just up and died. I did fished out the body and did a 25% water change.
Just yesterday, my rainbow shark's split tail suddenly developed fin rot, as did one of my guppies, who has had a torn dorsal fin since I bought it. Neither was particularly bad, so I moved them to a 10g hospital tank and added some (somewhat old) Jungle Fungus Clear. I had to chase the shark around for a quarter hour before I could net him, so he was a little beat. Last night, both fish were a little lethargic, but not dying.
This morning, I found the guppy dead, but the rainbow shark recovering somewhat (The white fungus stuff was going away, and it was swimming around). On top of that, my female guppy, which I think had fry, was dead. Once again, it was looking healthy just last night.
What do you guys think? What am I doing wrong? Why are my fish all dying?

I am a college student, but I have a house and an internship, so I can only monitor the tank in the evenings. I'm also notoriously bad at explaining, so please bear with me, and tell me if I missed anything important.

Thanks!

 

[N0body Of The Goat]

What are the dimensions of this 56g column tank? By a "column aquarium" I fear you have one of those ghastly tall but narrow tanks, which could explain a lot of your issues.

The Rainbow Shark and CAE need a tank at least 4-foot long, while the Angelfish need a tank with at least 18" of water depth (excluding the sand/gravel).

 

[JohnDyer]

I'm not sure on exact dimensions, but it is somewhere around 40" long by 15" wide by 20" tall, maybe a bit taller.
When you say 4 feet, do you mean 4 feet for each, or for feet to keep both together?

 

[JohnDyer]

Anyone?

 

[N0body Of The Goat]

CAEs and many shark minnows do get spiteful as they get older, the former tend to go for any tankmates (latching onto flacks, sucking slime and removing scales) while the sharks tend to get territorial with anyone of a similar shape, colour or anything invading their patch of the tank floor. Neither should be housed in anything smaller than a 4-footer and given I think you said that currently it is just these two fighting with each other, the CAE would be my one to rehome. The CAE may have been attacking the smaller fish, at some point it will be likely anyway, one of those fish that amazes me how many are missold to fishkeepers by irresponsible fish stores.

Your Tiger Barbs might be lashing out at other tankmates because of their small numbers, have you noticed anything going on?

At some point your Angelfish will regard small streamlined fish like Neon Tetras (that they eat in the wild) as "live lunch."

Is your gourami a male? These can get territorial with other mid/upper water dwellers, especially ones smaller than themselves.

Are you doing 25% water changes every week? Carbon should only be added when you have medication traces left after a treatment and a large water change, it quickly losses its adsorbing properties within days.

Have you got a liquid test kit rather than test strips, to accurately find the ammonia and nitrite levels?

Temperature could be a factor here too, 80F is awfully high for Neon Tetras and to some extent gouramis. Unless your Angelfish is an Altum, you could safely turn down the heater to 76F. Fish kept in too hot conditions burn themsleves out much quicker than their normal expected lifespan.

 

[JohnDyer]

Thanks for the quick response.

I've been looking around this forum and others recently, and the CAE doesn't seem like a good choice. How would I "rehome" it though? I'm not going to dump it outside, and I'm not sure if the local pet stores will take it back, being almost 6 inches long.

I've paid a fair bit of attention to the barbs since I noticed scratches, but they don't seem to be bothering other fish. I'll keep a closer eye out. They do, however, float head-down at a 45 degree angle on occasion, especially around noon and right after feeding. Is this normal?

My angelfish, unfortunately, is dead. But it generally seemed like a very slow-moving fish, while the tetras moved quickly. Do they get faster as they get larger?

My gourami is male. He's a slow-moving guy, but one again, I'll keep an eye out for aggression.

I've been doing 10% water changes every week, and 25% monthly. This last week, though, since my fish were getting sick, I've done 3 25% changes. The pet store people told me the carbon could be used for months, but the filter comes with a micron cartridge as well, and I've been switching it in for a week every month. The instructions say to soak it in 1/4 bleach for a night to clean it, then soak in water for 2 days and wash well. Does this work?

I've been bringing water to the pet store for nitrate and ammonia tests. I think they use the strips though. The liquid ones are expensive.

I honestly have no idea what type of angelfish it was, it was labelled as "assorted." I'll lower the temperature by a degree or two, though.

My rainbow shark, who is still in quarantine, has been clear of the whitish fin rot for a bit. Now it's developed a white spot about 0.5-1 cm in diameter. What should I do about it?