I would think that the Fluval 205 wouldn't be enough for a 37 if it's heavily planted. I like to over filter anyway... But, I would be going with a 305 on a tank that size. I have a 404 for my 45 gal. tank if that gives you an idea... When I changed out my substrate, everything went smoothly. I took out all of the decor & plants, put them in a rubbermade bin, drained most of te water, added fish. Emptied my tank, cleaned it out of all gravel & sand I had in it. Rinsed it thouroughly, & added a think layer of peatmoss, mulm, & then the (rinsed) Soilmaster. After that I added my decor (rocks & driftwood), 1/3 of the water (fresh clean water), plants, & then fish... filled back up with old water, turned on my heater, filter, & let it be. Now my tank is going very well, lots-o-growth!
Good luck, let us know how it works out for you!