tank problum

Question
i have a aquarium which is about 100liter capacity and ihave kept eight smill kind of gold fish but it become quite dirty in very fast time tell me a way how to keep water clin for a long time and i have a under gravel filter so plise try to give me easy way to takle this problum and if u can tell me also abou t one more problum i like little aquarium shark fish but i dont know it is possible to put gold fish and shark in a same tank so if i can then tell me what is the right temperature to kept them both. so plz try to send easy and affortable method to takle this  

Answer
Hi Arjun,
Thank you for your questions. The temperature should be in the high 70s fahrenheit. If you live in a warm climate with warm nights, you don't need a heater in the summer.
As to the compatibililty of fish. It depends. If you have an old timer in the tank he might attack any fish you try to add. If they grow up together, any fish might get along. If you get a troublemaker, try to find somewhere else for him to live.
The cloudiness in your tank is probably due to over feeding. Put them on a diet! Here is my page for wholistic tank care:
http://steamboats.com/aquarium.html
feel free to write back if you want to discuss further.
Nori