I have a new tank and I am a first time fish owner. The tank is 30 gallons and we have a whisper power flter 40. It says it is 210 gph. We have 2 ryukins that are about 2 inches and a koi that is about 3 inches in the tank right now. I plan to move the koi to the outdoor pond in the spring and get 1-2 more 2 inch goldies. Do i need another filter? Also..do I need to add a machine that adds air bubbles to the water? Thanks!

I can't stress enough how important is to measure oxygen in the tanks. Get a tetra kit for oxygen (costs around 4 dollars) and an ammonia kit and monitor oxygen and ammonia periodically. These tests will determine if you need extra equipment or if your current setup can sustain the additional fish... If you detect ammonia you need a larger filter, if your oxygen is below 5 mg/l you need to install an airstone or a larger filter. Hope this helps!