240gal ≈ 909L
2ppm in 909L would be a total of 1.818g sodium silicate.
To get 1.818g sodium silicate with you 37.5% W/W solution you would need 1.818g / 0.375 ≈ 4.85g of solution.
However, this also includes the mass of sodium not only silicates. You want 2ppm silicates. The required information is not in the SDS but apparently this is the commercial standard:
SiO2: ~28.7% w/w
Na2O: ~8.9% w/w
Density: ~1.39 g/mL
Which also fits approx. 37.5% W/W sodium silicate.
Therefore you would need 1.818g / 0.287 ≈ 6.33g
The density of the solution is not specified in the SDS making it difficult to calculate the volume in mL to dose. A quick websearch says it should be 1.39g/mL for the solution, so:
6.33g/1.39g/mL ≈
4.6mL
You need to add 4.6mL of the solution to increase silicate levels by approx. 2ppm in your 240gal tank.
I would still prefer someone to double check as I'm typing on mobile right now and typos happen fast