The timing doesn't really fit though, I would have expected all the fish to be struggling first thing in the morning, not mid day.
This could be consistent with increased CO2/low oxygen
Generally it’s assumed that in a planted tank with injected CO2 that there is an overall net production of O2
BUT this is really only likely in a tank densely stocked with fast growing (metabolism) plants
(re my earlier comment about stoned fish in planted tanks)
Looking at your recent photo, I feel that the carpet is still struggling & overall (especially, actively growing) plant mass is fairly low ... it maybe that “CO2 in” is outcompeting O2 production
Various algae’s are also known for sequestering oxygen from the water column rather than increasing oxygen availability
This is also true of most bacteria - a good reason to remove the muck and debris from your filters & tank bottom
dropped my co2 levels right down till the drop checker is a dark green / blue and still the fish were gasping at the top.
In contradiction with above, this suggests the issue may NOT be CO2 related
Of course in reality, there is going to be more than a single process occurring in the tank at any time - the overall net effect may still, simply, be reduced oxygen availability
If you’ve checked the Prime (lack of) effect with more than a single bottle of Prime, I’d suggest that ammonia is not the issue
BUT this doesn’t rule out nitrItes - you need 5-10X dose of Prime to sequester nitrItes (I don’t recall Seachem research details though Dr Greg Morin had released them on the APD ... this info was also available on Seachem's website back when it had the technical data pages and articles)
I also still wouldn’t rule out possible substrate effects based upon the email reply - ask them what tests are done before substrates are released to market, how tests are carried out, conditions of test tanks etc
Try to test water immediately when you see fish gasping at surface - unfortunately it’s not cheap/easy to home test for available oxygen but you should be able to reduce the chances of ammonia and nitrItes (I don’t know how comprehensive your ammonia testing has been, or how long you waited after Prime addition to conclude it wasn’t helping)
Despite some rather jaundiced views expressed on this website, I’ve found most test kit errors to be human operator rather than kit chemistry limitations
(I ran standard curves, “unknown” and test samples with multiple kit brands and chemistries - given the negativity expressed about hobby level kits on forums, I expected poor performance, instead most kits performed to specifications)
Hobby grade kits are not terribly accurate or precise but most are sufficient within 10% (& certainly 20%) of the predicted values
While
postulated tank ions and organic compounds MAY interfere, most are
far lower than the measured/documented interference levels needed for notable impact upon the chemistry methodologies employed by hobby grade kits - these particular (well documented) chemical methodologies were chosen for their impervious, stolidly dependable nature
😉
Of course one may purchase a faulty kit, but this is seldom the more probable scenario
Some (more expensive) kits also include positive standards by which kit chemistry (& your technique) may be checked
Kits that have been left open or stored incorrectly, especially after opening, are more likely to produce errors - depending upon the specific kit - one US hobby club asked several members to use a variety of kits testing several solutions (sorry I don’t recall which kits specifically) ... result, even with 2 yr old (open) kits, human error was the determining factor behind
incorrect results
btw I’m not suggesting that you go out and purchase a load of kits
😉
As this situation has been ongoing for over a month - I think - I’d be sceptical that this issue is biological “cycle” related
(it may be, but it wouldn’t be my first guess)
If you don’t have a nitrIte kit, I might pick up one of those, Seachem MultiTest Nitrite/Nitrate includes a reference standard - again I suggest reading through Seachem FAQ & website, perhaps even send your query into Seachem Tech Support (pm me first if you’d like and I’ll (likely) ask you a load of questions to clarify so that you avoid any rote responses
😉 )
