Under different circumstances depending on which type of fish species, there are several reasons on why you shouldn’t use sinking type for your middle and top dweller community fish. Foods that remain at the bottom of the tank will often get lost and missed out by your fish and thus gone wasted. Sinking pellets will also pose a problem especially those tanks that have large sized gravels because once the food touches the bottom bed; it will disappear into the gap. The foods that sink to the bottom will also most likely get sucked and trapped among the filter media and the condition gets worst especially if you have an undergravel filter system. Sinking foods that remain at the bottom of the tank will eventually rot and this will lead to proliferation of bacteria that may cause spreading of harmful fish disease.
When it comes to goldfish, it is a totally different scenario. There are some disagreements among experts as to whether floating or sinking pellets should be best fed to their fish. But basically based on my own experience, it is better if you use a sinking type instead because goldfish being a greedy pet will usually dash and swim towards their food and during the commotion while grabbing the pellets, they will end up sucking air into their stomach as they gulp their meal. Usually what you can observe is that air pockets will get trapped in their poo discharge and prolonged condition are sometimes blamed as the cause of flipping disease seen happening on goldfish. Therefore my recommendation has always been to strictly use only sinking food pellets for your goldfish.
As for koi, they are actually considered as bottom feeders and the reason why I’ve said so is because if you observe carefully, the fish has their mouths pointing downwards. This natural anatomy means that sinking pellets are best fed to them because it is easier for your pets to take a gulp of the foods. However, this doesn’t mean that floating foods cannot be offered to your fish as this will bring them up to the surface of the pond for your closer look. Koi enthusiasts mostly prefer to give their pet floating pellets because this is the time that they usually get the chance to view and get friendly with their fish. As for myself, I will use a combination of floating and sinking pellets as fish feeds for koi.