Paul,
I believe it can be safely re-dosed full strength weekly with no adverse effect. But see below for Virkon info.
Virkon Aquatic.
Virkon has been used in animal husbandry for some 20 years or so, but is a relatively new medication as far as the Koi industry is concerned. Why are we always the last ones to catch on? Like potassium and chloramine T, Virkon is a powerful oxidising agent, but there the similarity ends.
Virkon is a ‘modern’ disinfectant that is widely used in all forms of livestock husbandry, including pigs, cattle and sheep. It is also the disinfectant of choice in the fight against bird flu and foot and mouth disease, is licensed for use with all livestock and recommended by DEFRA (formerly MAFF).
Not only is Virkon a very potent disinfectant, but is less toxic than either potassium or chloramine T. It can be used in many different ways, as a foot dip, bath or spray to disinfect all kinds of equipment, buildings cars and clothing and can of course be used in direct contact with the animal in question to kill pathogens.
The manufacturers claim that Virkon is very effective against a range of viruses as well as bacteria, and in the Koi industry this will obviously be of great interest.
Virkon is effective at all temperature ranges from 4 deg C upwards, and is also long lasting in terms of its disinfecting action. Virkon only loses 10% of its effectiveness in water after 7 days. It is non toxic to humans when mixed, although like Chloramine T it is a respiratory irritant in its raw powder form.
When it does break down in water it leaves no harmful residues, so is kinder to the environment, and lastly it is licensed for use with fish, so will be readily and widely available.
Dose: between 2 and 4 grams per ton (220 gallons) see below. Dose is dependent on water hardness. Can also be used as a drip fed constant dose to keep bacteria count low. Can be used with salt at 1/2oz per gallon.
Water Hardness: Terms and Conversion mg/l CaCO3 German Degrees of Hardness Described as Dose per 220 gallons
0 - 50 0 - 3 soft 2gm
50 - 100 03-06 fairly soft 2.5gm
100 - 200 06-12 slightly hard 3gm
200 - 300 12-18 moderately hard 3.5gm
300 - 540 18 - 30 hard 4gm
540 + 30 + very hard 4gm
1 degree of hardness = approx 18 mg/l CaCO3
Virkon Aquatic is available in a handy tablet form to make dosing easy, as well as in the raw powder form.
more great info here copy and paste the link:-
https://www.koicarp.org.uk/medications%20and%20their%20uses.htm#Virkon_Aquatic
Tony 
nice one thanks for the link much appreciated