I have a Marineland 5 gallon aquarium, with 1 betta, 2 corys, and a snail. I tested the water this morning and found the ammonia and nitrite levels were very high. Did a 50% change and retested a little while later. The levels are lower, but still high (between 1-2 ppm ammonia and about 0.25 ppm nitrite). Should I do another change now, or should I give it a day and retest?