if your talking about dosing something like randys 2 part.... if you are then just have one tubing on each side of the tank away from each other and u should be fine.....
or like i do on my 29 get some milk jugs. i have a little stand built to sit above tank height that has a tube coming down to top water off, and i just poor partial amounts of 2 part into tank
You could manually dose the 2 part or just switch to kalk in your topoff water. You can probably get away with just kalk in a 28gal. As for the topoff, hide a freshwater reservoir somewhere and a Tunze osmalator is low profile enough to put in the main display.
I walk over to my 55 tank, test the water to see what it needs then I open the container, and measure the amount needed/req'd for my size tank. After all is measured properly I use the "Pour" Method, works great. usually I "Pour" in the general area of a powerhead and that helps to spread the solution.
it depends where you are dumping it. Like Chief said, dump whatever as close to a powerhead as you can and you should be good. slower pours also will help dilute it faster as well.