APT Complete should be the default choice. It provides the optimal balance of nutrition and algae control across a wide range of tank styles. It contains a complete set of nutrients necessary for plant growth and does not require additional supplementation.APT Zero contains no nitrates and phosphates, and should be used if you have a fish population that produces adequate nitrates in the tank (specifically, If your tank measures >10ppm NO3 by end of the week with no fertilizer dosing and water change).APT Zero can also be used to induce steep nitrate limitation in tanks, which gives the deepest reds in species such as Rotala rotundifolia and Ludwigia arcuata. This approach is usually paired with a rich substrate to prevent long term stunting due to low nitrogen levels. To this end, APT Zero can be used as a replacement for the ADA range of liquid fertilizers (Bright K neutral, Green brightly mineral and Green brightly iron) to similar effect.Estimative Index would be your choice if you understand and seek to practice the EI approach to grow plants at a fast speed. EI has the highest risk of runaway algae issues if you have sub-par tank maintenance, sub-optimal CO2 levels or low plant mass.