poorly fish???

Question
over night my fish has red (blood in colour) streaks through the base of its tail and on the top of its body. I know my water had needed changing & i saw that old food was sat at the bottom of the tank. What do you think the problem is? many thanks. The tank holds 23 litres & has 4 fish, 2 medium & 2 small.I change the tank once a week to every two weeks & use clourine neutraliser for the water which also coats the gills etc.

Answer
Hi Katie
Red streaks is generally a sign of one of two things.  Since you mentioned there may be a problem with the water quality that would be my first guess as to what's going on, ammonia present can cause the red streaking on the fish.  I'd recommend picking up a good test kit, I recommend API Freshwater Master Test Kit, it has all the tests you need(ammonia, nitrite, nitrate, ph, and high ph) for about 30. at the pet store, about 15. if you buy it online(those are US prices).  Ammonia and nitrites should always be 0 ppm and nitrates kept under 20 ppm-for an established cycled tank.  And, assuming there is ammonia present right now, I would do daily water changes for the next few days till that level falls to 0 ppm.  

I just did a conversion on your tank size, and that's equivalent to about 6 US gallons....what kind of fish do you have?  It's hard to say without knowing what type of fish you have, but it may be overstocked and you may need to think about a larger tank.  Usually the only thing I'll recommend for that size tank is a betta, or maybe a small school of smaller fish like neons.

I would recommend you increase your water changes and do them weekly, changing about 25% of the water each week, and be sure you're also doing a good gravel vacuum when you do that as well.  Don't overfeed the fish, feed ony what they'll eat in a few minutes, then net out anything that's not eaten.  When the food is left in the tank, it'll cause problems with the water quality.  

I really think that's what the problem is in your tank, the water quality.  But, there is an internal bacterial infection called septicemia that causes red streaking on the fish as well.  But, I would suggest doing the increased water changes and seeing if that helps first.  Not sure where you're located, but usually for septicemia you need a medicated antibiotic food, and I know that's not available to people overseas.

Hope that helps and good luck!

Christy