I'm sorry to say you haven't been changing the water nearly enough.
Is it a fish bowl or a filtered tank? You need to change about a fourth of the water in a filtered tank once a week. You need to change all the water in a bowl weekly or about half the water every three days or so.
Did you check for ammonia and nitrites? You only posted a result for nitrates. Whenever you have a sick fish you need to check for ammonia, nitrite, and nitrate. Post these three results on the board. (You will get more responses and they will be more helpful if you do; many of us don't know what to say when a poster gives insufficient information.)
How about testing Spike's water right now, as well as your five gallon tank and post up the numbers just for practice and so you can get a good idea about the health of your other two tanks. Then you can get some good advice regarding your tank maintenance so that you will hopefully be able to avoid having other fish deaths.
The woman who told you that keeping a betta in dirty water will make it live for seven or more years is a fool. Bettas who live for four or more years are the ones that are well cared for, with frequent water changes and proper feeding. (Once a day is usually enough feeding.)