I don’t have root tab at the moment. Can I inject the substrate with liquid ferts? I saw lots of store selling syringes and needle for this.
No. The benefit of good quality substrate tabs is the very slow release of nutrients to the plant roots. Flourish Tabs do not release nutrients into the upper water column, which is why they benefit plants like swords but do not encourage algae. Liquid fertilizer dissipates into the water, and in excess can cause problem algae.