My betafish is acting strange and getting a rusty color.

[wackywocky555]

I have a betafish who has lately been acting very strange. He'll suddenly start chasing my black mollies or my neon tetra's. He has killed 2 neons and bitten a big notch in the tails of the mollies. Some of the mollies have been bitten more then once. My beta is also starting to get this rusty color on his head and it seems to be getting bigger (the rust, not the head).
I only have a Nitrate test kit so I can't tell you the PH level and the other stuff. Sorry. My Nitrate level is 12,5.
My tank is a 60 litres aquarium (about 15,6 gallons).
My aquarium is a Juwel Aquarium and it came with a built in filter. The Juwel website says that it pumps 400-1000 litres per hour. I last did a water change 2 days ago. I changed 60% of the water since my aquarium started to stink. My aquarium is 6 months old and so is my beta. He was my first fish. I've been feeding him with OCEAN FREE Super MINERAL, but I changed to Tetra TetraMineral 2 weeks ago because the pet store near me stopped selling OCEAN FREE Super MINERAL. I think thats all the information I have.
Please help. My beta is my faverite fish and I want to know whats wrong with him.

 

[hacket]

Bettas can be unpredictable, even if you think you know them. What seems to be a peaceful betta may just turn on it's tankmates. My opinion is that all male bettas should be kept alone, because after all, they are animals and have aggresive instincts.

Could you get a picture of the 'rust' forming on his head? Also a test of all other parameters is a must in determining water quality.

 

[Islandlady]

Male Betas are territorial and some are more aggressive then others and like hacket mentioned above, they can be unpredictable. Also only a few types of fish are really safe to keep with a Beta and Neons and Mollies really are not on that list. Though it is your Beta that is attacking the other fish, Neons are also known to be nippers and so they could cause damage to your Beta's finage. Also, if your tank is over stalked it could be part of the reason. A weekly water change of 25% or up to and no more than 50% should be done in order to keep the fish healthy. There should be no bad smells coming from your aquarium.
When it comes to food, a variety of foods is preferred and is healthier for the fish. As for the rusty coloration on your Beta's head, I haven't got a clue? I have a male Beta and he lives peacefully with Harlequin Rasboras and Otocinclus Catfish. These are two fish species that are known to better get along with Betas! He is my second Beta and my first Beta who was more territorial would on occasion take chase after my Harlequin's, but no harm was ever done and it appeared to be more of a game, as my Harlequin's never seemed to really take him serious. lol
I haven't got a clue as to what the rusty shade on your Beta's head is and hope you can get him and your other fish back to good health! Good luck!