יורה דעה פ״דYoreh De'ah 84

Se’if 1א׳

שְׁרָצִים הַגְּדֵלִים בַּמַּיִם שֶׁבַּכֵּלִים וְשֶׁבַּבּוֹרוֹת, שִׁיחִין וּמְעָרוֹת שֶׁאֵינָם נוֹבְעִים, מֻתָּרִים אַף עַל פִּי שֶׁאֵין לָהֶם סְנַפִּיר וְקַשְׂקֶשֶׂת. לְפִיכָךְ שׁוֹחֶה וְשׁוֹתֶה מֵהֶם, וְאֵינוֹ חוֹשֵׁשׁ לִשְׁרָצִים שֶׁבָּהֶם אִם יִזְדַּמְּנוּ לְתוֹךְ פִּיו. אֲבָל אָסוּר לִשְׁאֹב בִּכְלִי וְלִשְׁתּוֹת מֵהֶם (אֲשֵׁרִי פא"ט וְאָרֹךְ כְּלָל מ"א וְכָל בּוֹ). וְאִם פֵּרְשׁוּ מִמְּקוֹם רְבִיָּתָן, כְּגוֹן לַאֲחוֹרֵי הַבּוֹר אוֹ עַל שְׂפַת הַכְּלִי מִבַּחוּץ, אַף עַל פִּי שֶׁחָזְרוּ, אֲסוּרִים. וּמִסְתָמָא אֵין לָחוּשׁ שֶׁמָּא פֵּרְשׁוּ. אֲבָל אִם לֹא פֵּרְשׁוּ אֶלָּא עַל דֹּפֶן הַכְּלִי מִבִּפְנִים, מֻתָּרִים.

Insects that grow in water that is in house vessels, pits, and caves, that don't have fountain in them, are permitted, even though they don't have fins and scales. Therefore, one can lean over and drink from them, and not to be concerned with them, if they happened to come into his mouth. (But it is forbidden to draw [water] in a vessel and drink from them (Ashiri and the Arukh Klal 41 and the Kol Bo) And if they left their place where they grew, like outside pit or on side of vessel, even though they came back, they are forbidden. And presumably, there is no place of concern that they left. But if they did not leave but are on side of vessel, on inside, they are permitted.

Se’if 2ב׳

הַגְּדֵלִים בַּמַּיִם שֶׁבַּחֲרִיצִין וּנְעִיצִים, (פֵּרוּשׁ חֲרִיצִים, אֲרֻכִּים וּקְצָרִים נְעִיצִים, רְחָבִים כְּגוֹן אוֹתָן שֶׁעוֹשִׂים לְבִּיבָרֵי דָּגִים, רַשִׁ"י), שֶׁהֵם מוֹשְׁכִים וְאֵינָם נוֹבְעִים, יֵשׁ אוֹסְרִים וְיֵשׁ מַתִּירִים.

Those that grow in water that is in furrows or ditches, (explanation: slits, long and short ditches, wide like made for fish inclosures, Rashi), they are withdrawn and not flowing, there are those who forbid and there are those who permit.

Se’if 3ג׳

הַמְסַנֵּן מַיִם, אוֹ שְׁאָר מַשְׁקִים, וְהָיוּ בּוֹ תּוֹלָעִים אוֹ יַבְחוּשִׁין אוֹ יַתּוּשִׁים, אַף עַל פִּי שֶׁחָזְרוּ לְתוֹכוֹ, אֲסוּרִים, שֶׁכְּבָר פֵּרְשׁוּ. לְפִיכָךְ מַשְׁקֶה שֶׁדֶּרֶךְ לִגָּדֵל בּוֹ תּוֹלָעִים, וְכַיּוֹצֵא בָהֶן, אֵין לְסַנְּנוֹ בְּקִסְמִים וְקַשִּׁים, בַּלַּיְלָה, שֶׁמָּא יַחְזְרוּ וְיִפְּלוּ לְתוֹךְ הַכְּלִי, וְיָבֹא לִשְׁתּוֹתָן. הַגָּה: אֲבָל מֻתָּר לְסַנְּנָן דֶּרֶךְ בֶּגֶד אוֹ מִסְנֶנֶת, שֶׁהֲרֵי לֹא יוּכְלוּ לִפֹּל דֶּרֶךְ שָׁם אֶל תּוֹךְ הַמַּשְׁקֶה, וְכֵן מֻתָּר לְעָרוֹת הַמַּשְׁקִים מִכְּלִי אֶל כְּלִי, הוֹאִיל וְהֵם תָּמִיד עִם הַמַּשְׁקֶה, הַיְנוּ רְבִיתַיְהוּ (בֵּית יוֹסֵף).

The one who filters water, or the rest of liquids, if it had worms or gnats or mosquitoes in it, even though they returned inside, it is prohibited, because they had already left. Therefore a drink that worms normally grow in them, and similar situations, one does not filter it with wood slivers and straw, at night, lest they will return or fall inside the vessel, and one comes to drink them. Rema: But it is permitted to filter through a garment or sieve, for behold they are not able to fall through there into the drink, also it is permitted to pour a liquid from one vessel to another vessel, since they always are with a liquid, this is sanitary treatment (Beit Yosef).