I think your test kits are not giving you the right answers.
Several large fish in a small tank, few and small water changes, no live plants...
...means your nitrate will be way too high.
Fish hanging at the surface may mean they are getting too little oxygen. This can be because there is reduced oxygen in the tank (Warm water holds less oxygen) or because the fish is otherwise compromised.
Ich can live in the gills. Your treatment might have killed the Ich. At that high temperature Ich goes through its life cycle so fast that a week of treatment might do the trick. Might not have, though. Watch out for a resurgence of the Ich parasite.
Fungus or other disease might affect the gills, and the stress of being sick and the stress of dealing with medication can also cause what looks like breathing problems in the fish (Lethargy, hanging at the surface)
If one test kit is old, another might be, too. If the nitrate is old, and no longer registering (Based on tank care as reported) then the nitrite also might not be accurate. Nitrite can enter the fish's blood and make the blood so it does not carry oxygen well. Then the fish responds as if it cannot breathe well, hanging at the top, lethargic. Are these Aquarium Pharmaceutical brand name tests? There is a code on the bottles. The last 4 digits in the code are the date (month and year) the product was bottled. Most of these tests are good for at least 2 years, many for 3 years. Some of mine seemed to work OK at 4 years, but the results became questionable, no color change in some tests. Comparing the results with other test kits, and knowing my tanks as I do, I am able to say the old test kits were not accurate any longer. There are ways of checking the accuracy of your test kits, using some of the materials that are used by planted tank people to fertilize the plants in the aquarium.
Here is what I would do:
1) Begin two-to-three times a week 25% water changes (even daily, if you can), being really sure to do as deep a gravel vac as possible each time. Maybe you can't vacuum the whole tank each time, but if you can do half or a third of it each time, then over several water changes you will get it all done, then repeat... keep on vacuuming. Be sure to lift any caves, rocks, driftwood or other things, and vacuum really well the substrate under them. Poor water circulation in these areas can lead to a build up of certain toxins.
2) Add an air bubbler or power head to increase the surface movement of the water, doing the best you can to circulate the water from the bottom of the tank so this most oxygen depleted water sheets across the surface to gain oxygen and lose CO2. Your filters already do this, with the intake fairly low in the tank, and the release point at the surface, but I think this tank needs more water movement. Make sure whatever you add works with the existing equipment to enhance the water movement.
3) Clean the filters in alternation. One this week, the other next week. Rinse the filter media in water removed from the tank for a water change and reuse the media. (Never rinse in tap water that has chlorine or chloramines.) Lots of beneficial bacteria in the filter media. If you are using activated carbon, rinse it really well before adding it to the filters. Keep the media good and clean for the best water flow, both for the benefit of the fish, and for the benefit of the beneficial bacteria that live in the filters.
4) Get a new test kit (even if it is just the test strips), or get a sample of water to a store that can confirm or not your test results.
26 tanks, over 3000 liters of water to keep clean and fresh.
Happy fish keeping!