Simon, there are several buffer stabilizers on the market, go to your LFS they will have some. They are called "ph strabilizer" or "buffer up" Low buffer means that if there is any change to the water, your ph can suddenly change too. I have low buffer in all my tanks, and it hasn't given me any problems. ..but again, buffer stabilizers are available in pet stores.
I did try nitra zorb, it didn't do anything! I cost me 15 us dollars and I've retuned it to the store. My problems of continuos nitrate and nitrite spikes were solved by purchasing a new better filter.
I was always advised against putting sea shells in my aquarium, for different reasons: 1. you can introduce new desease into your aquarium, and if it is a SEA shell, salt won't really help in sanitizing 2. it will leak calcium, and will change the ph and the hardness of your water.
wow...for now, add water conditioner to the tank. I hope the demage is reverable... add aquarium salt..follow direction on box. That may help repair some damage. Your waterchange should be enought to do once a month with such a big aquarium, and especially that your fish are so small, as long as you don't overfeed. Feed only twice a day maximum, and only as much as your fish will eat in 2 minutes.
one more thing about your water change..how big is your tank and how many fish you have in there? I clean my 20 gallon with 3 small goldfish once every 3 weeks. ..and only take out 20% of the water maximum when you do routine water changes. It is less stressful for the fish. Also, always add water conditioner before you add the water.
Actually, I was researching bettas on the net and went to newsgroups, etc, because i originally wanted to get one in my empty 5.5 gallon tank. Bettas will not live long in temretures bellow 75 degrees farenheit, and they really prefer tempetures from 78-80 degrees. If your beta only lives couple of weeks or months, it's not normal. Their life expectancy 3-5 years. If they are in cooler waters than desired, it demages their immune system, and will die a lot sooner. Goldfish should not be kept in such high temretures.
I use plecos to eat alge, they are peaceful but see a lot less of them. I just heard not long ago, that after they grow to a certain size, they stop eating algae alltogether. They can grow to be 38 inches!! I think in aquariums, they stay at 18 inches, max.