The mean temp of Lake Malawi is 77'F. and changes very little, because it sits right on the equator. So a few degrees on either side would be an optimum temp range for cichlids endemic to it. And because mbuna or any other Lake Malawi fish have evolved to those very stabile conditions, any temps too far off on either side are enough to cause stress, which compromises a fishes immune system.
Tilapines are an entirely different story, they range from the mid east where it gets cold to South Africa.where in winter temps dip down into the 50sF. So Lake Malawi fish and Tilapia endure vastly different climatic conditions. A tilapine endemic to Lake Barumbi mbu or Bermin would be exceptions and probably not tolerate the condition a tilapine like nilotcus from the ever changing waters of the Nile in Northern Africa, or Tilapia gallilius from Israel normally live in.
Ich range in size from 0.5 - 1 micron, this is small enough to pass thru just about any standard aquarium filter. And although it would not normally be found in treated tap water. The cysts ca be brought in on plants, bog wood just about anything previously used in an aquarium or pond.