Filling straight into your tank is perfectly safe but dosing for the complete volume is a must though. The reason for this is Chlorine and Chloramines are not the only thing that Prime will bond to on a molecular level. There are many chemicals,minerals, dissolved organics etc in our tanks that prime will actually bond to long enough for our biological filtration to take care of. The problem this makes for in tank water changes is when you dose for the amount of water you are replacing only, much of the Prime can be used up bonding to these leaving a lower concentration of the Prime to do its job with the Chlorine/Chloramines. Seachem have this info on their sites and also state on their bottles to dose for the complete volume if treating in tank. For those treating in a tub prior to introducing it to the tank, treating for the replaced amount is fine. If you are going to age your water first, no prime is needed if your water has Chlorine only as this will evaporate off fairly quickly with a little aeration or water movement. However, if your water has Chloramines in it you will still need to dose. Chloramine is used now in most municipal water supplies because it doesn't evaporate off like straight Chlorine.