The fact that this has been a recurring problem indicates that the underlying cause has not been found and dealt with. You say water changes helped in the past, suggesting this may be a water quality issue.
Purchase a liquid test kit (e.g. API master test kit) and let us know where your parameters stand. A 12.5% water change weekly is not enough with the large fish you have in your tanks. Your fish are also too large for the tanks they are in, which may be adding further stress.
The treatments you have used are targeting bacteria, while sliminess of the skin is most commonly caused by protozoa and flukes, generally secondary to poor condition of the fish.
For now, you need to do the following:
-If possible, test your water immediately and do a large 50%+ water change (test water prior to water change so you can see the true values).
-Purchase Seachem Paraguard and begin treatment. I recommend this because there may be a secondary bacterial infection causing fin rot, and it is best not to have to mix medications. Paraguard is also less stressful on fish, will help prevent secondary infections, and will clean and protect open wounds
-Maintain pristine water parameters throughout treatment. This means 0 ammonia, 0 nitrite, and <20 nitrate, and temp at about a constant 80F. The sump should be providing plenty of aeration. Test your water daily and perform daily water changes if necessary to keep perfect water conditions.
-If you find that your water parameters are the issue, make changes so that you don't have to deal with diseases like this nearly as often. This means larger weekly water changes, larger tanks if possible, and checking water parameters regularly to insure that what you are doing is working.