ACK filtering has been proposed as a technique to alleviate the congestion on the reverse path of a TCP connection. In the literature the case of a one-ACK per connection at a time in the buffer at the input of a slow channel has been studied. In this paper we show that this is too aggressive for short transfers. We study first static filtering where a certain ACK queue length is allowed. We show analytically how this length needs to be chosen. We present then some algorithms that adapt the filtering of ACKs as a function of the slow channel utilization rather than the ACK queue length.