So the plan is to have a FOWLR directly underneath the new DT & eventually move the sump to the basement? Might as well bite the bullet, move whatever ducting & plumbing you need out of the way in the basement and build a fishroom. Have one of the walls of the fishroom run perpendicular to your joists & the 2x4s will provide added support to your main floor. That's pretty much my plan, I already have the fishroom walls roughed in in the basement.
I prefer photos over drawings if possible, then there's little doubt about what the structure looks like. If you can save or convert a jpg photo to bmp format, you can mark it up using MS Paint. Then save again as jpg to keep file size reasonable.
I keep seeing the statement 'if the house is new...'. What's up with that? My place was built in the late '50s and as far as I'm concerned the floor joists & supports are more than likely better than some of the construction I'm seeing out there these days with 'engineered' floor joists. These things scare me, since they are thinner than solid wood joists and made of scrap wood wafers glued together. I sure hope that glue holds for 50 plus years, like my solid joists! Laid diagonally across my joists are ship lapped 7 inch wide solid wood planks and plywood/flooring on top of that. I find it's more about maximizing profit in construction now and many builders will tend to cut corners. Not saying that's the case with all builders, but let's try to look at each case individually rather than making sweeping inferences about home age affecting how we go about solving a problem. Anyhow, that's my rant for the day.
I would add my voice to those recommending you remove the tile under the tank. That area will be unseen anyway and you'll find it easier to open the floor for drain & return lines from the basement. Leaving them there will be a waste of tile. Replace it with a section of exterior grade plywood. I'd even take a little more tile away & have a border of some other material around the stand.
Here's a photo of my future fishroom in the basement. I haven't bothered marking it up with paint as per my above procedure, but it's pretty straightforward. Foundation wall on the right, which I primed & painted with good quality concrete paint. Left of photo shows the last 2x4 of the wall I put in opposite the foundation. Decided to use pressure treated lumber for the 2x4s that are the base of the walls laid along on the basement floor. I don't plan on any floods, but without a drain in the basement floor, if it happens, these will hold up better than standard lumber. My tank will also sit parallel to the floor joists & serve as a room divider between kitchen & dining room. So the wall on the left will add support & help minimize main floor deflection. Take into account that there is often a crowd of folks standing/walking around our display admiring it. Depending on the size of your friends, there's the potential of quite a bit of additional weight around your display. And if it's a high traffic area, continued flexion may contribute to material fatigue over time. No harm in overbuilding something, peace of mind is a good thing.
