Are YOUR TD's Wagons ? Some people use TD meaning other things...
If yours are wagons I don't know if you can effectively flush them in situ.
However, If you have a sedan I suggest that power flushing is completely acceptable way to really clean them out. I took my 240 tank out... but would not do that again because the large holes in the tank and the upright placement makes power washing and evacuation very straightforward....and easier than dealing with the very tight rubber interference gasket at the bottom.
If the only thing you are trying to do is kill algae.. you can do that in the tank with Biobar ... I did this ... and use a clear 90 degree filter up front.. and it only took one treatment per 6-9 months to keep me clear.