It is impractical to make the pond Holy using the ritual.
Assuming that the pond is perfectly cylindrical, it has a volume of 7,363 cubic feet. Google claims that such a volume is equal to 440,633 pints. All of the flasks listed on PHB 152-3 contain 1 pint, so if we assume that the ritual makes 1 pint of holy water, it would take as many hours to make all of that water Holy according to the PHB ritual.
A cleric of any level can spend at most 16 hours a day doing the rituals, because they have to spend at least 8 hours a day resting in order to get their spell slots back. At that rate, it would take 75 years and about 11 million gp worth of silver to turn the entire pond into holy water.
Unless I've made a serious math mistake somewhere, this order-of-magnitude calculation shows that it doesn't make sense to turn a large body of water into holy water.
However, the cleric does have the Divine Intervention class feature (PHB 59):
The DM chooses the nature of the intervention; the effect of any cleric spell or cleric domain spell would be appropriate.
It seems to me that making a pond full of holy water would be within the purview of this feature, though it's ultimately up to you as the DM to decide.
There are no rules for potency.
I could not find any rules that dictate whether or not Holy Water retains its powers if diluted. Therefore, it's probably up to you as the DM to determine this.
What's the point?
Currently, in the rules, holy water doesn't have too much importance; it's a somewhat weak attack against fiends and undead, and it's a component of some spells.
I think that it's likely that your cleric player wants to accomplish something specific with the holy water. You should ask him what he wants to do with all that holy water, and then work with him on how he can accomplish that particular goal, instead of getting hung up on specifics like this. I find that focusing on intents and goals, rather than whether particular methods are impractical or not, tends to have better results at the table.