Will any lawn fertilizer work? Won’t it be toxic for the fry?
Any lawn fertiliser will work. When the culture is green soup the algae should have used all the nutrients in the fertiliser and it is safe for the fish. If you are really worried, you can use aquarium plant fertiliser.
You can also test the green water for ammonia, nitrite and nitrate. If there is a reading, leave it for a few more days before using the water for the fish.
-------------------
Do not syphon the fish out.
-------------------
Coryodras do not shoot out spines or barbs or anything else.
When they are attacked or stressed out, they stick the dorsal and pectoral fins out sideways. The first fin ray on these fins is hard and pointy and the fish can keep them sticking out for quite a while. This is a safety mechanism the fish have evolved to stop other fish eating them.
There was a thread on here recently with a goldfish that tried to eat a Corydoras. The catfish stuck its spines outsideways and got lodged in the goldfish's mouth. Fortunately the owner of the fish managed to get the catfish out.
Some loaches (clown loach) have small spines on the side of their head and erect these when threatened or attacked.
-------------------
If you want to catch fish out of an aquarium, get 2 nets.
Have a 5-6 inch net and a 8-12 inch net.
Put one side of the big net against the front glass and rest the bottom of the net on the substrate.
Use the second smaller net to slowly guide the fish into the big net and then lift the fish out in the big net.
-------------------
Get an adjustable heater so you can set the temperature on whatever you want it on. Most tropical fish need the water temperature on 24-30C (75-86F) for their eggs to hatch.
Baby fish grow fastest at 28C (82F).