Hello all! Let me preface this by saying that I'm new to the hobby, having had my tank around a month and a half. Though new, I've thrown myself in headfirst, read every book, subscribed to all the big YouTubers I like and watched hours of videos and listened to days of podcasts, realfishtalks and so on. I researched every part of my tank and have driven my girlfriend mad with my newfound hobby. All this said, this is a pretty bad situation, and I know the value of experience, so I come to you good people with the hope that you will help me beat this threat to my aquarium. The tank in question is a marina lux 70 with stock lighting. Fluval stratum substrate, planted with jungle val, spiral val, some lovely crypts I'm yet to identify (green tops of the leaves with Lovely red/purple underside and Broad leaves, any ideas?) bacopa compacts, crypt parva, java moss on some spaghetti rock and a good amount of java fern on some driftwood and a ceramic ring, fertilised daily with 2.5ml of George Farmers, the Aquascapers Complete fertiliser. Currently conducting 50% water changes daily with Prime and feeding fish relatively sparingly. Parameters are as follows: Temp: 24.5 degrees celsius Ammonia:0.25 (could be down to api test as am using prime and believe I have managed to re-establish filter with only a mini-cycle) Nitrites 0 Nitrates: above 75 ppm ( tapwater contains 22.5ppm) Ph: 7.4 Filter is the marina i160 twin cartridge affair that came with the tank as I currently can't justify getting a decent hang on back to the partner, hotrodded slightly with bio noodles and some filter foam. The tank is fairly well stocked, with 3 young pearl gourami (1 male), 3 female peppered corydoras (all laying eggs everywhere before the catastrophe.), 2 oto's (did have 4) and 4 zebra danio (1 male,did have 2 males), notice the did? Yeah, onto that. Myself and my girlfriend went on a trip across the UK to visit family, leaving my stable and happy little patch of nature rigged up to a battery powered automatic feeder (twice daily). Imagine my horror on arriving home to the lights off, filter not running, and heater off. Somehow the power pack they were all plugged into(hindsight... lesson learned) fell off the chest off drawers it was on top of (again, lesson learned) and managed to trip the fusebox. I have no way of knowing how long the power was out, the trip away being for 7 days. My current guess is 3 days. This is because of two things, 1) one of my original 3 oto's survived, I found one floating and one dead on the bottom covered in what looked like the white tufts of columnaris mouth fungus or some other water mold. The other is going strong today, no hiccups 2) The water was bad when I came back, but not what I would have expected from say a 6 day blackout. Honestly I breathed a little sigh of relief because the tank wasn't covered in algae, only two of the most fragile of fish was actually dead on discovery and I was hopeful for being able to recover my tank from this situation. I cleaned off the front and both sides of my tank,scrubbed the algae and debris off the ornamental bonsai tree, trimmed some dead leaves off the plants, removed the fish, gravel vac-ed and did a 70% water change with prime. My filter uses a manufacturer supplied cartridge refill, two slots for two cartridges. Restoring power showed that the flow rate through the cartridges that had been stagnant for god knows how long was well below par, so I removed one, replacing it with a clean cartridge, hoping any surviving bacteria will quickly populate the new filter and then changed the remaining filter 4 days later. Onto livestock, immediately I inspected all of my fish, one Danio had a small white patch on his head between his eyes and above his top lip. All the other fish looked fine. Immediately ordered a breeder net online as a means to quarantine him away from the group. First two days he was fine, feeding and acting like a danio should, but on day three my breedernet arrived when I went to QT him I noticed the white patch had travelled much further up the head and that a small lump had formed on the edge of what I was now realising was not simply a "white patch" but necrosis of tissue, he was literally losing his face. QT'd and over the next day and a half salt dipped him around every 16 hours in 3% salt solution. Yesterday I noticed him going between gasping at the surface and resting on the bottom of the breeder net, so I gave him a 30 minute salt bath and hoped to see some improvement, but on returning him to the tank he stayed near the surface for 2 hours before finally rolling belly upwards and losing control of his swim bladder ( rolling over and over, on his side sometimes etc.) within 10 minutes of this he was dead. I've taken post mortem pictures of him, including cutting off a gill plate as I noticed him staying near the airstones before I had QT'd him, so wanted to checfk on gill tissue damage etc but other than what looks like a melted face, I couldn't see any obvious problems with the gills(no brown or white spots or dead tissue) This morning I've awoken to another of the Danio's listing from side to side, not able to quite control his bouyancy. One of my cory's is being EXTREMELY shy too, just sitting in the corner of the tank, and dashing away if you get too close before being a statue again. I've gone through every bit of information on the internet to do with fish diseases, and am still having problems identifying exactly what it is that is killing my fish. but my suspects at this point are Columnaris (though I can't say which strain as it features both the outward symptoms and a seemingly long incubation/mortality period.) or similar bacteria. I have seen no classic Cottonwool around wounds or mouth though and am having a of a time here. I've attached pictures of the danio who passed, as he is the only fish with visible external symptoms. I'm here all week monitoring this thread for both a chat about fishkeeping and for advice from you lovely people here at FishLore. This is the biggest threat to the little ecosystem that I call my own, help would be hugely appreciated! Thank you for reading this wall of text guys, Dan.