Oof… well, this can be remedied by reducing return pump flow! — if you reduce water input enough that a single drain can keep up, your redundant overflow should start working as intended!
— Regarding the new tank, the first step would probably be removing those black plastic plumbing parts that pass through the tank bottom (“bulkhead” fittings)…
— these are being removed because the rubber sealing washer (gasket) on each is likely quite old/dry rotted, and liable to weep water if put back into service!
here’s a diagram of what these bulkhead fittings actually looks like, for reference purposes:
— note the flange side (measurement A), it’s the part visible in your photos, with a gasket (measurement E) directly under it, between flange and glass…
— now, to the outside of the glass; the bulkhead nut (measurement D) is the part you need to unscrew! Some are reverse threaded like the above diagram, but many are standard threaded… if the nut isn’t loosening at all in the standard direction (counter-clockwise nut rotation), examine the threads an note the direction of spiral! (Note that the entire fitting might begin to spin once loosened, requiring a helper to hold the flange side in place, or unscrew from the flange side)
— Once you have these bulkhead fittings removed, we can confirm hole diameter(s), and decide if the existing bulkhead fittings need a simple new gasket, or if it would be prudent to replace them altogether!
