I would strongly recommend buying a test kit. Especially if you have that many tanks. PetSmart has master kits (ammonia, PH low & high range, nitrite & nitrate) for $22 or less. It is a small investment for a whole world of information & making sure your fish are safe and comfortable.
The photo you posted looks more like a flying fox fish than a shark, sorry. That may be incorrect depending on age, however we only have a photo of the back to tell.
It can take 10+ days for ich to appear. It is not uncommon to have fish that appear normal but are indeed carrying the parasite. Ich is a protozoan parasite. It could have been transferred to your fish through the water in its initial tank. Obviously we don’t know where your fish came from, but if a single fish in its tank carried the parasite, your fish will be affected- whether it is visible or not. If your fish are stressed, they are more susceptible to infection- something you would be able to determine with an adequate test kit.
Keep up with the water changes and I highly suggest adding Melafix or aquarium salt to save the fish you do have in that tank.
If you have recently added new fish to your other tanks consider raising temp and treating them as well if any are showing signs of stress (lingering at the top or bottom of the tank, red gills, etc.)
And absolutely get yourself a comprehensive water test kit. Your fish will thank you.
Your shrimp may not do well with the melafix, just so you know. I unfortunately lost a few shrimp treating tanks. They are very delicate.