We are not taking any sides here...
But do you think it is fair for the Buyer since it already stated to you that he need a certain dimension it fits but in actual fact it doesnt... It is the same as if the dimension is stated on the product box but the dimension of the actual product is not, it is a case of false information being given.. And the Buyer trusted you on that being that he is new to reefing, and yes, he should have checked when he was at your house and that is his negligence.. He was considerate enough not to call you at 12am midnight and waited till 9am when he found out it cannot fit.. And what loss have you got when he offer to bring it back to your house and offer for a refund.. As for the vulgarity during conversation, we cant be the judge different ppl have different tolerate level for vulgarity..
And clearly from the way you reply, you seems unconcerned about how the product is working for the buyer and seems only concern about getting the product off your hand..
Anyway,
Both party have their negligence in this sale, if after so much discussion and explanation, if the Seller still take his stand of not refunding of the money. The seller will have to find other alternative for the CR..