In the past 2 weeks, I have had 3 puffers die, 1 was a 2 1/2 year old GSP, we found him lodged in the filter tube

The other 2 were Figure 8 Puffers which we got this past Saturday. They were doing great, I put a frozen blood worm cube in last night which was bright red in color, but they were both kind of standoffish at first but then started to nibble. The morning came and before I left for work I checked to see what they were up too and they appeared to be fine. An hour later my boyfriend found they floating in the tree. He said that the blood worms were black in color and laying at the bottom of the tank. I'm kinda thinking it was the bad blood worms that killed my GSP, but I don't know! It's really upsetting, I really love puffers and this is breaking my heart!
1. Unfortunately I do not know the water perimeters but i am going out and buying a test kit today.
2. My tank is a 20 Gallon and there was only the puffers in it, and a large rock so they could hide and 2 fake trees.
3. I fed my puffers twice a day and they would usually get a cube of blood worms, I have brackish water, so I put the salt in, the cycle solution and a water purifier.
4. I have not made any changes to the tank that is out of the ordinary. Just the same routine I have been doing for 2 1/2 years.
5. Ive had the tank for 2 1/2 and have been doing the same routine and just recently my fish started dying on me !