Puffers should never be netted. See this puffer care article
Ich can kill but if it turned black it sounds like a Melanocarcinoma or a thyroid tumor and the slime could have been Brooklynella Hostilis. See www.fishyfarmacy.com
for more info on Diagnosis and Treatment of fish diseases.
Also read Selecting Healthy & Appropriate Marine Fishes
The use of a QT tank
while not mandatory is highly recommended as well.
is only needed in a QT for invertebrates preferably for 2+ hours. With fish, you need to get them out of the transport bag as quickly as possible. Test the transport water for temp, salinity, pH and so on. Then manipulate the QT parameters to match. Once done, the acclimation timeframe can be greatly reduced. Temp, pH and salinity being the main concerns. Bring the QT sg
/ph/temp up to the main tank over the 6+ weeks kept in QT and increase the sg
/ph slowly and every 3+ days to avoid stress.
See Help setting up a QT tank
& Equipment for drip acclimation.