Hi Nitrate/pH levels - what effect on my Barbs?

I have 4 Odessa Barbs in a 5gal tank (small, I know) with a power filter (sponge I think), rocks and gravel.

The fish were bought 3 weeks ago and I was asked to bring a water sample back to the place I bought the fish - to check things were ok. Shocked to see the water turn dark purple in less than a minute of testing and I was told to change the water (50%) immediately and feed them less (I was only feeding every other day but may have put too many flakes in). He then said to do another 50% change on Wednesday. In such a small tank, the food and waste have obviously created a high bio load (trying to use the correct terms here, still fairly new to fish-keeping).

My question is, how much damage to the fish would it do if they were in high levels for at least 3 days and no more than a week? And are there any other ways I can help to reduce the Nitrate/pH levels rather than just relying on frequent water changes?

thanks for the reply.

I will do a 50% change on Wed and another one on Saturday.

We were told we can add another 2-4 Barbs (as the like bing in schools) to the tank once the water is at acceptable levels. But on seeing the posts and getting advice from others on here, a 5gal tank is a bit small, even for 4 barbs, so surely adding anymore would overcrowd the tank and make the bioload even more troublesome?!

I don't think you would class this place as a LFS. It is an accredited (UK) fish keepers and the people in their seem very knowlegable... when looking to buy the fish, the bloke went round showing us the fish he thought would be ok in a small tank and showed us the effects of having fish in too small a tank (from fish that people had brought back from their homes). So I believe this 'store' care for the fish and you have to remember they weren't going to allow us to buy any more fish with the condition the water was in and told us to go home and change the water immediately!

With the one inch per gallon rule (which I've heard about before), I would say we have just enough room for the 4 and will not add any more barbs/other fish.

Another Question

I will post this on a sep thread if I don't get much of a response but..

Should I have lighting for my tank if I am keeping barbs? And do they prefer a dark background to hide in (ie paint the back wall black or place a dark image behind the tank? At the moment, the back is a bright picture of plants and sand.