Welcome Will. If you are going marine it would be best to direct any questions etc to those in the saltwater section as most of the salties don't read the rest of the site.
Iodine is not the free flowing agent added. They are separate.
Did you know Roman soldiers were paid in salt? That is where we get the term "Worth his salt."
The word "salary" comes from it too.
Rock salt is mined from the ground, unlike our table salt (from Garssmere) which is harvested after evaporation.
They do form a gravid spot.
The fry will eat crushed flake.
Many livebearers do not like being put in a breeding trap and will often abort of moved into one. They are better off in a small tank with lots of plants.
As to when the fry go into the main tank - when they are big enough not to fit in the mouths of the other fish
Peas are good for constipation. Cooked and with the shell removed. They will go mushy when put in the water.
The reason they don't grow as large is because the small aquariums stunt their growth and this is not fair on any animal.
A number of fish are friendly when young but get more aggressive as they age.
Could be both but most likely constipated. When pregnant, they get fatter at a steady rate (like humans), they don't suddenly double in size overnight (although I am sure I did with my second ).
Are you sure she hasn't dropped her fry? That would explain the vent change. Guppies can drop fry every 4 - 6 weeks like clockwork. They get bigger and the vent gets bigger and darker until they drop. They fry will get eaten very quickly in a community tank unless there is plenty of plant for a few of them to hide in.
Poo colour will change depending on diet.
Afraid the pic is too fuzzy to be of help.
People spawning egglayers do so in bare tanks with grids for the eggs to fall through or these plastic plant mats on the bottom of the tank. I think I remember someone saying they were sometimes available in the $2 Shop or similar.
Difficult for us to help as different filters are found in different countries, sometimes woith different names. Are you looking for an internal or external filter? Not heard of the one you mention.
If people praise the good shops regularly others will soon work out which are the bad ones without their names being mentioned at all and lawsuits avoided