Sick Fish, what is happening?!

A while ago we bought a Gourami from the LFS we have issues with. They replaced it with some plants but the Gourami had a disease we failed to catch upon purchasing that quickly developed into a major tank disaster. We lost a few fish and waited about 2 weeks to stabilize the tank. Now everything seemed fine until yesterday.

Yesterday we purchased 3 clown loaches, which seem to be adjusting well. Here is the tank specs and I've got a few ideas as to what could be causing the issues now.

4 Zebra Danios
1 Swordtail
1 platy
3 clown loaches
1 apongeton
3 Red Ludwigia

As of today 1 anubias, 1 driftwood w/ java moss, 1 mondo grass..

30gallon,
ph 7.5
temp 79' (too high?)
0 ammonia
0 nitrites
10 ppm nitrates

Haven't done any water changes in a week as things seem stable.. There was a bit of dead plant matter on the filter and on the biopad which we rinsed in tank water, we use 1 whisper 40 and 1 elite 10 submersible, 150watt heater

The infection that the Gourami carried was visible as two small dull spots which scabbed over and fungused after one day in the tank, two days later it was dead. We didn't have time to find an open LFS after we noticed the fungus as it was a weekend and (our fault) we were quite busy.

The gouramis disease killed 1 swordtail, 1 zebra danio, and 1 columbian shark. The columbian shark was probably stressed to begin with as this tank was too small for it and as bad as it sounds I am glad it passed as it was a poor investment.

There is slate in the tank which we ran water tests on before putting in, the ph has remained absolutely stable throughout this entire disaster. Ammonia has never spiked, although the fish nazis at the LFS (who hate us for not taking their advice such as: its ammonia. yet tests prior to and after bringing water to them showed 0. I know this test is not old as it registered ammonia during cycling fine. also they advised us against doing regular water changes during fishy cycle as this would "get rid of beneficial bactera" something i have been told is bunkus).

The swordtail looks fine. It was eating a few days ago, very active, very normal behaviour. Now it sits near the bottom and sulks, no clamped gills (the other fish showed this while stressed during fishy cycle. not happening here) looks like it is sleeping, like how sometimes the fish will sit at the bottom and rest when the light goes off..

On a side note, the fish nazis were very mocking and rude when we went in, although we have been nothing but polite about the problem and not even asked for compensation although their disease obviously caused much of our heartache. Today I noticed they are treating several tanks! Yet somehow this is our fault.. no fish dies in 3 days from stress, sorry I can't see that. We've never had Ich, we've never had any problems before that Gourami. If the fish were stressed you'd expect outward signs of illness PRIOR to the Gourami dying... Maybe it was the change in PH? hmm, but he KNOWS our ph, why not warn us? anyway. They made up for it, and no soup for us anymore. ever.

Any advice? I do NOT want to treat for something I am unsure of. I don't want to unduly stress the fish when unnecessary!

We used to add salt on the advice of the fish nazis, but ppl here say not to. I trust these forums more than them. We have found another nonbigbox LFS to shop at where the staff are very friendly, knowledgable, and eager! Also the one bigbox we shop at has a great manager who couldn't be better really. Knows the fish section like the back of his hand and has great advice for us!

The sword was preggers when we got her and she gave birth, unfortunately she only ended up providing live food... Whats going on?!?

I should point out we are noobs, so if we are wrong we're more than ready to admit it.

I have to bump this, but I'll add that the apongeton LFS sold us is doing fantastic. The ones in his tank however are almost dead... likely because of the heavy treatments he has in some of his tanks right now...

I did give a full tankmate list. but I will again go over what is most relevant, and ask that anyone who thinks I may have missed something read over the initial post as it should contain anything I don't include in this secondary post
Ammonia: 0 Nitrite: 0 Nitrate: 10ppm Temp: 79 pH: 7.5

In the tank during hell: 1 Flame Gourami (new), 2 swords, 5 zebra danios, 1 Columbian Shark, 1 cute little platy

Died from disease: 1 Flame Gourami, 1 Sword, 1 Zebra Danio, 1 Columbian Shark

In the tank now: 1 Sword, 4 Zebra Danios, 1 cute little platy, and 3 BRAND NEW clown loaches.. also some snails!

Here is the symptoms: Notice 2 smal dull spots, dull as in clear on the scales of Gourami when purchasing. Thinking this was just from an attack or something.
Gourami's two spots turn slightly yellow and begin to scab, next day some white fungus on them, next day dead. No noticeable clamping, no heavy breathing, etc. IMHO this is Columnaris.. No mouth fungus I noticed though. Perhaps wrong.

The sword had some fin rot before he died which quickly became fungal. Columbian shark did not show any signs of disease until it was belly. the danio which may have but was small and did not seem ill until dead.

Now for the Sword that is sick right now.. NO outward signs of disease, very sedentary, no clamped fins

Re: Eating, the only time the fish havent eaten through this is right now with the Sword. We feed high protein flake and the occasional section of frozen blood worm. Also we have on occasion provided romaine lettuce or a pea. This is rare.

Absolutely. wish I could find some help but here is an update:
The temp was lowered to about 77 over the course of the day, that seemed to bring the sword back. I have read that perhaps they will only tolerate temps near 80 for a short period, this may be the issue as there are no other signs of illness or discomfort. Still not eating right now, although the fact that it is swimming and no longer sitting at the bottom is a good sign. no rubbing or anything, and it is still somewhat stationary but no fin clamping and it has moved to the middle/top region of the tank.

About the clown loaches, absolutely. These ones are just kids but we have another 30g to move them into (by themselves) if they begin to outgrow the community tank. We also plan to invest in a MUCH larger tank in the future but I am under the impression they grow slow, so this will probably not happen until we move into a larger place. We expect to be house shopping in 2 years, and selling this place. At that point we will setup a permanent large tank, probably a community tank 100+ that is if we don't get discouraged like so many do, but I am not one to give up! seriously i'm as stubborn as a mule this fish tank will never be torn down

as it is sunday i can't pick up a small quarantine tank, but this is probably the best idea. we will do this as early as possible. We have a 10 gallon submersible filter so we may choose a solution with no light, no substrate or plants, and simply fill a small 10 gallon drop in the sword and the filter, do you think we would need an airstone for such a small space with one fish? We can easily connect our existing air system to accomadate a second tank.

Okay I am just bumping this again. I have observed her all day, and she has not gotten much better. She finds a part of the tank and just floats. She was right above the 2nd filter, submersible Elite 10, and she totally ignored the flake food dropped right over her head. As for the quarantine tank we have settled on purchasing one tomorrow. I did mention above that she was previously pregnant, her gravid spot is still dark, and until the die off there was a male swordtail with her although I do know this is not a necessity for her to give birth especially since she was impregnated previously. Could this have anything to do with it? The temp levelled out at 77, should I lower it again? I am not inclined to do this as the heater is a simple 150w with a dial and it is not hooked up to a digital thermometer or cutoff circuit.