NEVER use Tetracycline (or any other antibiotic) in the pond WATER. Such medications may be effective if FED to the fish in a medicated food, but not in the water.

Secondly, ask the shop owner who gave you that advice to show you his copy of the book which recommends this treatment method. Try to buy it, because the book is surely an antique with collector's value.

Please don't put antibiotics in the pond water.

The reason is this, purification of water is dependent upon the activity of beneficial nitrogen reducing bacteria that reduce ammonias and nitrite chemicals into relatively harmless nitrates. When you added these antibiotics to the main system, you killed alot (or all of) the bacteria that purify the water. Obviously your intentions were good, but this massive die off of filter bacteria creates a problem more severe than a routine bacterial infection of the fish, namely, high ammonias and nitrites will result from the use of these drugs in the main system. Now you have to do major waterchanges to correct the ammonia problems, and the fish suffer for it. Futuristically, you can use drugs like these in a bath. OUTSIDE THE MAIN SYSTEM.