Actually, the severums would have been fine at a pH of 7.0 IF the changes were done slowly. Most of the fish we get are not wild caught, and have been bred to handle a winder range of pH levels. But, as Voodoo mentioned, changing pH quickly (more then .2 per day) can have extremely detrimental effects. Hopefully your severums are hardy lil buggers and can handle the change. Also, changing pH without knowing your hardness and buffering levels can be an issue too. Do you know them?
As for why William asked what size tank, you'r right, it has nothing to do with the pH. It does have a lot to do with the amount of bio-load in the tank. Are they bristlenose plecs? Or common plecs? Do know bristlenose will get to be around 6 inches, and thats ok for your tank, but common plecs get to be about 2 feet long and they grow FAST. And the tank size makes no difference. Tinfoils get to be around 14 inches and really need buddies as they are schooling fish, Your severum will reach around 6-7 inches. Do know your tank is way too small for all these guys, and you will likely have water probs in the very near future. Be aware you may have to return some of these guys, or purchase a much larger tank. In the meantime, be prepared for lots of water changes and testing!