Seachem equilibrium is basically CaSO4, K2SO4 and MgSO4 with some Fe added. This would increase the GH. Depending on the size of the aquarium and your willingness to experiment you could buy individual salts resulting in less expenditure and more flexibility in dosing. CaCl2 and MgCl2 can also be used to increase GH .
Anything with carbonate in the name will increase KH if it is water soluble. Sodium bicarbonate (NaHCO3) has great solubility, CaCO3 not so much. Raising the KH with carbonates will increase the pH but also “stabilize”/buffer it.
If you want softish water I would start at 25ppm Ca, 5ppm Mg, 3 dKH. When CO2 is being added, KH is not that important for plants so match for fish demands. Generally, most plants prefer a higher GH, I aim for 6 dGH but want to go higher.
Dose for the volume of water changed. After you calculate the dose for that volume, add and test to see if you got what you want. Test the first few times to make sure your dosing is accurate. If you dose with individual salts always assume highest hydration levels (eg MgSO4*7H2O )
I like to mix outside the aquarium in a large container, add a powerhead and dissolve almost everything before adding to the aquarium. This also prevents the high, low, high rodeo during water changes if you add pure RO and then add salts.
Hope this helps.
On hiatus till later this year