I read the previous five or so threads on this topic and am disappointed we still don't know what is causing this white line to appear on our cats. I have some anecdotal info to add that sort of runs contrary to an assumed premise, so FWIW...
I bought two S Eupterus in 1990, and still have them. Yes, you read that right. They are 24yrs old. One became dominant as is normal.
They lived in a 60g tank for the first 11 yrs of their lives. During this period the thinking in the hobby was that "aged water" was better and so maintenance was different back then. I changed maybe 10g a month. I never deep-cleaned the gravel to pull out the acidifying waste beneath the UG plates. The nitrates were naturally high, and the tank had short, green brush algae on the decorations and back of the tank, while I'd scrape the front and sides. The nitrates had to be sky high.
The water here is moderately hard (16dGH) and pH is 8.2 with KH 6, but I always kept the tank about 7.4 (used Seachem Acid Buffer to prepare water changes and the tank's equilibrium was 7.4 with the acidification keeping everything stable there).
The fish had a tankmate I bought at the same time (1990), which outgrew the 60g in the 11th year, so I moved all 3 fish to a 250g. The tankmate was a Distichodus Sexfauciatis or 6-Banded African Nile, and was very peaceful and loved swimming with the non-dominant cat, so he never picked on them.
By the time they went to the 250g in 2001 the thinking in the hobby had changed, and aged water was considered bad, along with UG filters, mulm under the plates, etc. The emphasis was on constant vacuuming and water changes, and keeping nitrates under 10-12ppm.
So I did not put a UG in the 250g and had to vacuum daily as the fish waste would float on the gravel. I was changing 10g water/day keeping the water parameters constant as they had always been (using acid buffer), but obviously nitrates were being kept ultra-low. And no algae was ever in that tank. (We can assume the TDS was lower too though no one checked back then.)
Well the cats loved the room to swim, BUT they both got white lateral lines for the first times in their lives! How odd it should happen when I was keeping everything so sparkly clean!
Flash forward 1 year later and their tankmate outgrew the 250g and I had to take him to SeaWorld where (after a quarantine period) he went into a 5,000g tank with other FW fish. At the same time I had to move and had no room for the 250g, so I had to sell it and move the cats back into their 60g.
Although I put the UG back into service in the 60g, I kept it very clean this time. I was on this new regimen of bi-weekly water changes of 8g and deep-cleaning the gravel to remove all mulm under the plates, etc. I also started using RO in water changes to bring down the pH rather than acid buffer. But maintained the same basic parameters they'd had (except for the water becoming a little softer from the RO, but considering it started at 16dGH or so, it was still about 8-10dGH.)
Meanwhile, the white line they developed while in the 250g, persisted. I try everything to get rid of it (not meds, but foods, etc) and nothing worked. I was frustrated that I was keeping everything so clean and they had this issue.
A few years passed with no change in my routine or in their white line condition, then a family member got a prolonged terminal illness and I become the main care giver for the next 5 years. For the last 3 of those 5 years I become completely overwhelmed and unable to care for the fish at all, except for feeding them and changing the BioChemZorb every 6-8 months or so. I did ZERO water changes, I didn't vacuum and I didn't clean the Fluval. They were virtually neglected except I was still observing them and could see they were fine, but I was not actively doing any maintenance for THREE YEARS. The pH was still 7.4, but the nitrates had to be off the scale.
My family member passed and I turned my attention back to my hobby and couldn't believe my eyes... THE WHITE LINE HAD GONE AWAY!!! COMPLETELY!!! Here I had tried so hard to keep them clean, and they 'got diseased' -- then I totally neglected them and they got healthier than ever!!!
So what did I do? I rewarded them by SLOWLY in phases, cleaning everything up. Over a period of a month or so I gradually vacuumed all the mulm out from under the UG plates, I cleaned the Fluval out, etc.. and (gradually) eased into using RO water with water changes instead of acid buffer to bring down the pH to 7.4 (in the change water, to match the tank).
And guess what happened after a few months of living in a pristine environment again? Yep. The WHITE LINE RETURNED. The dominant cat has it worse than the other. One side has a solid white line but I could only take a pic of his other side that has a couple blotches and a line by his tail. You can also see some gray circles along his jaw. That's new to this occurrence of it. They never had that before. http://postimg.org/image/t0swygla1/ (The blue line and vertical lines below the blue line are reflections on the glass, not on the cat.)
So contrary to this idea that cats need ultra pristine environments, it seems like in my particular case for whatever reasons, with these fish and my local water conditions, that my syno's do better with aged water, acidifying mulm under the plates, and sky-high nitrates! And I am going to reduce the water changes to 10g 1x/mo and stop deep-cleaning the gravel, letting that acidifying material build under the plates... and see if this white line doesn't go away again! (And yes, it takes a long time!)
I realize this is anecdotal and I am not certainly not extrapolating or promoting poor tank care. For one, let me remind any readers that this tank is way over-filtered. In fact 6 months back the Fluval started making a noise so I retired it after 24 years, drilled my tank, and put a 25g sump with a 3g homemade biofilter on there and a 350gph Dolphin pump. So it has the UG with two large powerheads, the giant Whisper with the best chemical filtration (trying Purigen at the moment as it is inert and rechargeable but I have BioChemZorb on hand as always) and the sump. Plus is it not overstocked.
But I am starting to think the old guard idea about aged water is maybe better after all, as long as it's purified by some good media. It's just hard to argue with experience! Especially ones you weren't expecting! I never expected them to get a condition in the ultra-clean 250g.... then I never expected them to get well during a period of no water changes or maintenance... and certainly didn't expect them to develop the condition again when I cleaned things up! Obviously, there's something there!
Anyway... FWIW this has been my experience with them over the last 24 years. I will find this thread again and post results if this forum allows waking a thread up after so long, but I won't know for awhile if my theory is correct. I am curious though!
Thanks for reading, to whoever made it to the end!
