with a split fin there is a greater chance of infection. clean water decreases the stress the fish may see.
remember, when we use tests to test water we use nitrates as a gauge. it is not as difinitive as it may seem. what we are actually removing when we do water changes is to reduce the TDS(total dissolved solids) nitrates are the indicator. in a planted tank you may not see nitrates or you may see low nitrates as the plants consume the nitrates but not some of the TDS. so water changes are uses to remove these dissolved solids we can't see or test for(without spending a great deal of time an money)