Diagnosis Assistance Needed...Maybe

[Gervahlt]

Tank Information:
55g - 1 Gold Severum (Heros efaciatus) @ 6", 4 Firemouth (Thorichthys meeki) @ 4", 3 1/2", 3" and 3", 1 Common Plecostomus (Pterygoplichthys pardalis) @ 7", 2 Texas (Herichthys cyanoguttatus) juveniles growing out @ 2 1/2" and 2".

Temp: 79 F
pH: 7.2
Ammonia: 0.50
Nitrite: 0
Nitrate: 40

^ using API Master Test Kit. Right after taking this reading, I did a 60% water change. I have not re-tested yet. Last week's test before water change did not show ammonia and only showed 30ppm nitrates. I usually do 30-40% water changes twice per week. This past week was busy so it's been 5 days since the last WC. I have not seen ammonia since the original cycle for this tank approximately 2 years ago.

Filtration: Emperor 400 w/ filter floss in both secondary media containers, AquaClear 30 with filter floss instead of active carbon

Possibly pertinent items in order of importance:

1) Texas juveniles were added roughly 4 weeks ago without quarantine since I didn't have an open tank at the time.
2) The AquaClear replaced an Aqua-Tech 10-20 roughly 1 week ago. I'm guessing this is probably why I'm seeing a temporary ammonia spike this week.
3) Feeding heavy at the moment to promote rapid growth of the juveniles. I feed Hikari Gold and Top Fin pellets daily, an algae wafer every two days, and frozen blood worms 2x/week. By heavy I mean that the pellets usually take about 8 minutes to be completely eaten.
4) Rocks shown in attached video added roughly 3 weeks ago.

Recent behavior: severum showing clamped fins, twitching, not being her normal aggressive self at meal time today or yesterday, but still eating, and hovering up by the powerhead where water movement is highest.

My main question for diagnosis: is the severum's behavior purely a reaction to the .5 ppm of ammonia, or is she (I believe this is a female) showing the signs of a parasite infection?

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In the above video, there is a bump on the right side just behind her gills that I noticed for the first time today. I tried to have the bump in profile to show size and placement.

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In this second video, you'll see the severum's recent behavior. Some small signs started yesterday and have rapidly increased to what you see here.

Thank you in advance for taking the time to assist me!

 

[PostalPenguin]

I would test for ammonia again because ammonia shouldn't be present and if it truly is, that might be relevant to helping diagnose the problem.

 

[Gervahlt]

Ammonia zero when I tested again at about 11am (about 11 hours ago now).

When I got home a couple of hours ago, a couple of the other fish had ragged fins. The raggedness didn't look like a result of aggression though, so I decided to do a salt bath on all fish (except the pleco). In the process of catching them all, I took out all the rock and other decorations and found a bunch of organic matter inside holes drilled into the plaster "driftwood". When I pulled it out, I accidentally ran it under the output from the AqauClear and it flushed down into my tank. It was worse than having an HOB back flush into the tank! Even though the water parameters are fine at this point, I'm doing a thorough cleaning to get rid of the organics as I've read that too much dissolved organic matter in the water can affect them even if all other parameters are good.

I'm still not sure about the ammonia spike. It could have easily been a false reading since I'm using the same kit to do a fishless cycle using ammonia on another tank...it's possible some ammonia didn't get fully rinsed off after the last test.

Either way, I'm going to be keeping a close eye on them and I'll update if there are any changes. I lost another severum about a year ago to symptoms very similar to those shown in the video. No treatment that I tried had any effect whatsoever. I tried jungle parasite medicine, heat, salt, quick cure, maracyn and maracyn II (all at different times and waiting in between each medication period). He lingered for months gasping for breath and doing his best just to hide before he finally couldn't eat. I eventually had to euthanize him.