Repeated Fish Injuries - Who or What is Causing Them!

I recently redid a 46 gallon bowfront tank of mine that had already been up and running for a year. The filter is a Filstar xp2. Water parameters are Ammonia/Nitrites/Nitrates: 0, pH 7.4.

The new fish have been in the tank for about a month, but I have had many fish "injuries" since I added the new inhabitants. I'm trying to figure out what the culprit is.

Fish injuries so far:

1.) The first was a pearl gourami losing one of its feelers, I didn't think much of this since it happens to them somewhat frequently, and it was so soon after adding them to the tank I thought the feeler may have been injured when I got it.

2.) A few days later one of the corydoras suddenly developed a paralyzed/injured looking left pectoral fin and died the next day. It was unable to balance itself and swim properly, though it had looked fine during feeding earlier in the day.

3.) A couple days later one of the pearl gouramis developed a paralyzed/injured left pectoral fin and died the next day. It was uninterested in eating although it was able to balance and hover near the top of the tank. It seemed mostly unaware of anything going on around it.

4.) 2 days later a corydora has a partially missing tailfin and the back end of its body (about 1/2cm of the tail connected to the fin) looks whitish and paralyzed, as though injured or swollen (or possibly even missing some scutes). The fish is listless and is having trouble swimming.

Current inhabitants are 6 juvenile paleatus corydoras (1" current size), 5 pearl gouramis (Trichogaster leeri, 2" current size), 1 rubber-lipped pleco (2" current), 1 horse-faced loach (3" current, and yes I am sure this isn't a long-faced loach), 2 apple snails (one the size of a small marble and one the size of a large marble), and some Malaysia trumpet snails.

Some of these fish will be moved into other larger tanks that I have, they are simply growing out right now.

Tank ornaments are small-leaved plastic plants (live plants are in the mail but haven't arrived yet), small-grained onyx sand, and a terra cotta pot with a 3" hole knocked in it (and sanded smooth) to act as a cave. Other things in the tank include the filter intake & outtake and the heater.

Any suggestions on what is causing these repeated injuries?

I attribute the lack of nitrates to the rooted philodendron stems I keep in my tank plus water changes. I actually had the water tested 3 times at 2 different fish stores in the past week just to be sure my test kit (and theirs) was reading correctly, so unless all 3 of us are using outdated kits in synchronicity, I think it is accurate.

I haven't witnessed aggression among the gouramis, though that was my first suspicion. I've kept other species of gouramis in the past but I was under the impression that Trichogaster leeri is generally a lot more peaceful than they are. The tank is in my bedroom where I spend the most of my time so I watch it often at frequent intervals throughout the day. It's possible they are being truly sneaky, but I'm beginning to think it must be someone/something else.

Only two of the fish had swimming problems, and this was directly after their left pectoral fins became injured (oddly the corydora and the gourami both had injured, semi-paralyzed left pectoral fins and seemingly no other symptoms than the slightly torn fins which they refused to use much if at all). The swimming balance issues seemed to stem from a mechanical issue with the fin they weren't using as opposed to a swim bladder problem. It was fairly similar to if you use the paddle only on one side of a canoe... you end up going in circles.

The gourami with the injured feeler is otherwise doing fine, and the corydora with the injured tail is doing poorly, I'm considering euthanizing him. The tail isn't white in a fungal way, it is just paler than the rest of the corydora's body and slightly swollen and stiff looking. It almost looks as though the corydora's tail/spine snapped just behind the dorsal fin so that it is unable to use the back half of its body. The back half looks like a recently dead fish with rigor mortis set in while the front half looks healthy... it's weird. Part of the tailfin itself is missing, as though picked off by another fish (or snail?) All of this happened last night or today, as they all were healthy during feeding last night. I've taken to counting everybody daily because these constant injuries are making me so paranoid.

Could the snails be attacking the fish as they sleep? They certainly must sleep soundly if that is the case. I've witnessed an apple snail try to munch a corydora during the daytime, but the cory instantly startled and swam off.

I sort of hate new tanks, it seems like it always comes with a time of uncertainty and speculation as you "debug" the system. I like it when everything falls into place and you have beautiful, healthy fish that thrive for years.

Could ich have something to do with it?

I hate refuting everyone's guesses because it makes me sound like I'm not being open to help... this particular issue has been driving me crazy because it's mostly a bunch of "what ifs".

In regards to the loach, I'm not going to say it is impossible, but it seems improbable. My three reasons for this are that the injuries began before I got the loach last week, horse-faced loaches are supposed to be peaceful tank inhabitants (and indeed I have had them in the past with no problems - skunk loaches aren't supposed to be as unaggressive), and the third is that he isn't the largest in the tank. He may be the longest, but he is really skinny, skinnier than a pencil. He has a small mouth and spends 99% of his time chewing on gravel bits (he sucks it into his mouth to sift for food and it falls out through his gills). He completely ignores the existence of all other fish he encounters... he isn't very threatening.

So... maybe... but probably not. I'd be more likely to suspect the snails of opportunistically munching their tankmates than him. And in their case it is unlikely, too, though I'm sure they would have no qualms eating anything that sat still long enough.

I've come up with yet another theory at least for the fin injuries (but not the corydora with the half-paralyzed body). A new symptom has appeared - ich. The first few spots have started showing up on the fins. I'm upping the temps and adding salt, but in watching a few of the fish flashing I began to wonder if they gave themselves their injuries in their rapid scrapings against things.

Also, today when I changed the water I lifted the heater another inch or so above the tank gravel. It's positioned horizontally near the bottom so I don't have to turn it off during water changes. It was close enough to the bottom that if food got underneath it the gouramis would turn sideways or struggle to reach underneath and get it. I figured if it was higher they wouldn't bump into it and potentially hurt themselves. The heater is one of the plastic-cased unbreakable ones, but it still has reasonably smooth edges.

I just have no idea on this corydora.

It all seems like "maybe, but probably not". Arg!