Recently into goldfish. A few questions.

1) My small tank around 15-18 litres feeding 9 gold fish. All small size, around 3-4cm without tail. Is the tank big enough? If not, how many litres of tank that I need?

2) The shop that I brought goldfish recommend us to buy Tension Gon and Taiyo PSBIO (Photosynthetic Bacteria). Is that necessary?

3) Currently feeding flakes type of fish food. Is that good enough?

4) My tank always look dirty even through I put a big hanging filter? Any idea to make it clear and clean?

Thanks first for all the answer.