Fish dying one-by-one

Hello, as the title suggests, I've had four fish (one swordtail, one apistogramma, and two cory cats) die in the past few days and I have another two (cory cat and dwarf gourami) who are not looking good.

Tank specifications:
Tank size: 55g
tank age: two months
pH: 7.6
ammonia: 0
nitrite: 0
nitrate: 10ppm
tank temp: 80F
(none of these parameters have changed since adding the fish... I checked when adding them, when the first death happened, and yesterday).

Fish Symptoms: The fish have done different things before dying.
The cory cats have all swum around like they're having a seizure (spinning around and crashing into plants), and then gone between laying on the ground/floating at the top and swimming like normal before eventually dying.
The swordtail didn't do anything special that I noticed, I just found it dead (it must have been dead for a few hours at least because I found it with a cory eating its stomach).
The apistogramma didn't eat a single time after I got her, and I found her dead four days after acclimating her to the tank (this is the first fish to die)
The dwarf gourami looks bloated which seems to be a common late-stage symptom of an infection gouramis get which is almost always lethal.

Volume and Frequency of water changes:
Since the tank is still pretty new, I have been doing 20% changes just less frequently than once a week. My last water change was four days before the fish started dying.

Chemical Additives or Media in your tank:
I used api quickstart and tap water conditioner when doing water changes, as well as aquarium salt.
I just got api stresscoat as well and added that right after finding and fishing out the dead apistogramma, which was the start of all the deaths.

Tank inhabitants:
The remaining fish and their time in the tank are two angelfish (six days), two dwarf gourami (a month), five cory cats (six weeks), three swordtails (a month), one apistogramma (six days), and one clown pleco (six weeks).

Recent additions to your tank (living or decoration):
The angelfish and apistogrammas, as well as one valsaneria and one ozlet sword (both plants). I added them six days ago, which was four days before the deaths started.

Exposure to chemicals:
None that I know of? Other than what I added during the water changes and the stresscoat, which I listed above.

Digital photo (include if possible):
I didn't get pictures of the first fish that died, but after calling my LFS yesterday he recommended the only way to get any more idea of what is going on was to supply pictures to a fish forum, so I got pictures of the swordtail that died yesterday (picture after fish was in the trash for a few hours) and the dwarf gourami dying right now. I haven't noticed any spots or abnormalities with them except the gourami since he is bloated.

Other thoughts:
I've done a lot of Googling to no avail this week, but the one problem I suspect now is that I have the fish tank light on a timer, but it doesn't have dimming functionality. I've seen some results about fish being stressed out from the light turning on/off suddenly, in which case that is probably not helping here. I've ordered a dimmer online now, and I'm going to manually dim the tank (it has a dial attached to the light) and ignore my current timer until I get the dimming one.

Thanks in advance for any ideas! I really just want to save my lil guys

80 seems a little high... but not to be like killing off fish. Everything else seems normal based off what you described. Did you try to rinse the media in your filter during The water change? And if so did u use declorinated water for it? I’m not sure man. That is a mystery to me

You mention.aquarium salt....why and how much are you adding?

I ask, because typically you would use this for treating something. Otherwise, there is no real need to add this to a freshwater tank. Maybe a very tiny amount to add back electrolytes and minerals, but I'm talking a pinch or 2. Prolonged use at therapy levels, or anything above that pinch once in a while, can and will kill fish. It can cause kidney failure (bloating), osmosis issues ect.

I didn't touch anything in my filter before, but just now I replaced the foam insert and rinsed the new one out thoroughly with dechlorinated water before adding it to the filter.

In response to @Fishiemang, I added the salt when I did water changes per the directions on the package (it says one tablespoon per five gallons added, so two tablespoons per 20% change in my tank). Honestly for no reason other than doing this the last time I had a fish tank (years ago as a kid). If I shouldn't be adding aquarium salt, that's great to know and I won't add it again. I really hope that I didn't kill my fish off by adding it.

Would you recommend I do another water change (I didn't want to leap to doing so since I just did it less than a week ago and the fish are stressed enough with all the death going on), so that I can reduce the salt levels?

Smaller 20% water changes to get the salt out. This brings the level down slowly. Don't add any more fish for while. You were dosing at therapy doses. Plus 2 tablespoons every 20% change and.your salinity is probably off the charts. When changing water, you add for what you subtract, so 20% of 10 gallons is 2 gallons so 1/3 a tablespoon is the correct amount, roughly, for that water change...when treating the tank. This creates a marine environment, not really good.for freshwater (not all fish like brackish water....some breeds can handle it like mollies and stuff...scaleless fish you run into issues quicker).

The salt may very well be the key here. What you describe is some of what happens.with prolonged use.

I agree. Salt should only be used in freshwater tanks temporarily for treating specific conditions. it should not be used routinely.

You mention using salt years ago - this was common several decades ago. It is possible that fish keepers back then found that salt kept their fish healthy - as we now know, salt mitigates the effects of nitrite, and perhaps they had nitrite in their tank water since the nitrogen cycle was not understood back then?